edit:
@Kamwah
Afraid so!
I guess the builds I'm losing to (or at least the way the player goes about using the build) are ones that aren't as prevalent in the higher leagues because they're easily countered and nullified with good use of skills such as storms and force-fields, or sometimes in the case of zerg simply opening with a stargate. I have no trouble with unit composition, or judging when to engage or not - it's just mis-micro that's doing me.
For example, most zerg players I play seem to mass roach, either going for an early bust or simply trying to get up 3 bases and hit me with 200/200, and I'm guessing they do so because it works against all the other Protoss players they come up against. A mass roach army is essentially A-move, with either running away or burrowing being the only other things the zerg will ever have to do, so it's not really a strategy that requires much skill to pull off when it comes to engaging. If I A-move a similar sized Protoss army (which it wouldn't be anyway, as my units are more expensive and I have to spend more on tech, so naturally I'm behind in supply if we haven't traded off and are on a similar macro level), I will be crushed. The pendulum swings back in the favour of the Protoss, especially in the early game, with the sentries - a good split makes it a piece of cake to clean up and I won't really lose too much myself. If I suck with sentries (which, relative to somebody like MC, I do), I'm going to struggle a lot more with the roaches than I should, and I'll probably lose my higher value army to his without him breaking much of a sweat.
Of course forcefields (and storms later on) are quite basic micro in the grand scheme of things, and I'm sure people who don't play Protoss will tell me they're OP or whatever ("you try hitting a good EMP", "you try splitting marines"), but they are absolutely key to surviving against most of the builds used against Protoss these days. It's simply botching of engagements that's costing me, whereas I don't really see what bit of mis-micro the mass-roach player, or a terran with a big MMM ball, can do to screw up an engagement on a similar scale, other than the act of deciding whether the engage or not in the first place. When you get to a higher level there are obviously ways in which players will do to get more out of their roaches than just try to hug my army, and there will be terran players who don't simply stim and a-move their ball into mine and generally hope for the best, but trust me when I say these builds are being quite effectively used at my level without the need for much micro ability whatsoever. Played about 15 games against terran in the past couple of days and I don't think there's been even a single attempt at a drop in my base.
FWIW I'm Plat, although hovering quite low at the moment after losing more than I've won in the past couple of days.