I think that it's a matter of belief, just like working magic is. For some reason, wizards have a tradition of not trusting tech to work properly, and so it doesn't. Normal people have a tradition of not believing in magic, so even if Harry walked them through a spell step-by-step, they probably couldn't get it to work. They'd ground out whatever power they managed to raise before they even became aware of it.
Butters is more open-minded than most, after all, he classed the vampire corpses as "humanoid but not human" before he knew Harry. Most doctors would probably try to explain the differences in the bodies as deformities or the effects of disease.
Ramirez is probably a good example of a wizard that is a little more open-minded towards tech. Part of it is his youth, but it may also be something intrisic to his personality that makes him just not worry too much whether tech will fail on him, so by default he trusts it more.
Early on in magical training, an initiate has to believe that all the symbols, words, and strange materials have innate power, and he is just putting them together like a puzzle. Later on, he comes to realize that all those things are just foci, and amount to nothing more than a way to get the conscious, worrying and doubting mind to shut up long enough to let the magic work. The foci are mental toys that get the logical mind to stop asking "but why?" for a while. If you were to reveal that the foci are nothing more than a psychological crutch too early in a mage's training, he probably wouldn't trust in those tools long enough to develop trust in himself.
By the same token, wizards are taught that tech is unreliable, but each one has a different line defining what's "simple" and "complex" tech. A sword could be forged with modern metallurgical techniques that might not have been available before WW2, but it still seems "old", tried-and-true.
I think that game-mechanic wise, there could be a few different options, or ranks of "tech friendliness", and you could spend some of your development points towards that. If Harry is a default, then you could say that if you don't buy or sacrifice any ranks of "tech-friendliness" you're comfortable with things that predate you by roughly two generations (or about twoscore years
). You could make an effort to adapt over time (Ebenezar obviously has, he's around 300 and the fact he can work
any motor vehicle is a wonder in itself) by spending experience-based development points on it as well.
If the default rank 0 is about -40 years, then the next rank up could be -20, then 0, then +20, +40, +60 and so on. Normal humans probably default to somewhere around the +60 level if they grew up in a developed country, as that would be about the age where all the "newfangled gadgets" start to seem too mystifying to use.
After 5 increments of 20 years each, you could start taking larger steps. Maybe it could increment by 50 years after that? Ebenezar is able to use a truck that postdates his birth by about 200 years, so that would put him at 7 ranks of tech-friendliness, even though he's still using very old stuff by modern standards.
Oh, and on the side topic of generating "shields"...
From:
http://www.scansite.org/scan.php?pid=203Industrial Force Field
Industrial Case Evidence
An invisible force field has been experienced as a by-product of a particular industrial process in a 3M manufacturing plant in the south eastern United States. This occurred in August 1980 at its polypropylene plant around a large film-slitting machine with usually a temperature of about 80 degrees Fahrenheit with a relative humidity of 75-80%.
David Swenson of 3M Electrical Specialties Division in Austin, Texas was called in to investigate the problems with contamination of wide web film as it was being run at high speed, converted (split) into “film jumbos” with a width of 3’ x 5’ for coating with adhesive to make tape.
The Polypropylene web was 21 feet across – with the film running from one roller up 20 feet to another roller; across 15 feet to a third roller; down 20 feet to a splitter; and was then wound onto two rolls. It formed a huge dynamic “tent”. Swenson was there to measure the static electricity inside the web tent . As he approached the tent, his static field meter recorded a 200kV @ 12” The amount of static electricity was in the Mega volt range. The force that engaged him inside the tent was invisible and impenetrable, certainly making him unable to move further forward.
This was Coulomb’s Forces – the law of static attraction and repulsion – Static Electricity can cause some very strange and often difficult to explain phenomena. It is easily controlled if a commitment is made. This nuisance can be reduced to a non-nuisance level by proper analysis and installation of suitable equipment, at a very low cost.
To get rid of the effects at the plant an induction static eliminator was installed across the web at the unwind. The static electricity was immediately reduced throughout the process to less than 50kV (from MV’s). It was then possible to make measurements in the rest of the area. Additional induction units were also installed at the jumbo wind-up areas, which resulted in less than 5kV at conclusion.
Another link on this:
http://www.esdjournal.com/articles/final/final.htm 
