Low FG

I started all grain brewing a couple months ago and i am having trouble with my FG? The last 3 batches I have brewed all finished at 1.02-1.025 area instead of the final gravity of about 1.01-1.015. I use beer smith to build my recipes and have hit my OG and FG on multiple occasions but lately I cannot get mine to finish completely. I have a temp controlled freezer I use for fermenting, I have it set for 67 degrees with a 3 degree differential and it holds a pretty constant temperature. Does anyone have some advice as to why I sometimes reach my FG and sometimes I don't?

Probably need more info on the recipes/process for the problematic batches. Could be any number of things - mash temp, yeast type, aeration, malts, etc.

What are you using for a thermometer? Thermometers are notorious for being off.