There can be several different things that happen to change your seam allowance. Here are a couple of the main things.... if you have to change your needle, make sure you get the same size needle that you have been using; do all of your sewing on the same machine (changing machines can cause your sewing to be off); press your blocks, don't iron them (ironing is moving the iron back and forth...pressing is putting the iron down on the seam and then picking it up again).
I do a lot of measuring. When I sew two blocks together as a set, I check out the measurements to make sure that they are coming out the correct size. If I need to, I trim the set of blocks before I sew them to the next set.