Is some corn sugar more fermentable than others?

I've been noticing lately that my beers are not are carbonated as well as they've been in the past. My last bag of priming sugar was bought at Williams and when I ran out 2-3 months ago, I bought a 5lb bag of corn sugar at my LHBS. My beers have been coming out a little flatter than anticipated and was wondering if it could possibly be due to the change of corn sugar?

The sugar should be the same and ferment equally well. It is possibly however that one suppliers source (Williams) was finer particles and the other (LHBS) was coarser so when you measure them out (assuming a measuring cup) you can actually fit more by weight of the finer particled one in the same given volume. Corn sugar is typically pretty fine, not as fine as powdered sugar, but much finer than table sugar.

You should measure by weight, not by volume, additonally, did you put the same amount of sugar in this batch as the last one? Was the first batch fully fermented? Some styles require less carbonation, so take that into consideration, all corn sugar should be the same by weight, so it could be that you didn't let the bottles sit for long enough or they didn't fully carbonate for some other reason, of which there are many,.

I measure by corn sugar on the digital scale I use to measure my hops. I'm using Beersmith to calculate how much corn sugar to use and usually let them sit 6-8 weeks before I start worrying about under-carbonation. I have no idea why my pale ale is a little flat, but maybe the American wheat and Summer spiced ale may be a little under-carbonated because I used Wyeast American wheat 1010 on both those. I noticed that those beers tended to ferment slower so that may be the reason its taking longer to carbonate - but wouldn't 2 months at 70-75 degrees be sufficient?