I play more or less only one table sit-n-gos. I have been staying pretty much even bankroll-wise, but lately I've noticed a disturbing trend. I've been getting knocked out by going all in preflop with medium to low pockets and getting taken out by higher pockets. This usually happens when the table is reduced to 4 or 5 players.
There must be a reason for this, but I can't figure out what. Am I overvaluing the pocket pair? At a shorthanded table with big blinds, they are often a way to get an unfair flip in my advantage or steal a decent pot preflop. So what's going wrong? Should I not be going all in? What about position? Is this just a very dragged out unlucky streak?
Any thoughts would be much appreciated.

you don't know why your lower pocket pair is consistently losing to a higher pocket pair?

Well, the times you don't get knocked out by a higher pocket pair, nobody calls your bet and you get the blinds. People don't call you when they have crap, so the times you get called, quite often the other player is going to have a high pocket pair.