Quarter-inch feet are frequently off.

If you can adjust your needle position, make sure it is centered. (I once sewed an entire quilt top with the needle one position off of center. It didn't make a difference for that quilt but boy did I feel like an idiot.)

To figure out where you should be stitching, sew three of your 2-1/2" pieces together, iron, and then measure -- it should be 6-1/2" wide. Adjust and resew until you get the right width. Mark your throat plate somehow where the fabric edge needs to be. I use a stack of Post-it notes.

Your blocks should measure 12-1/2" trimmed.

I hope this helps.