paddingtonbear

The same used to happen to me. It sounds like your problem could be a combination of inaccurate cutting and inaccurate seam sewing.

You may be beyond this but I found it so frustrating. I overcame it by buying a good long plastic quilting ruler, a rotary cutter with new blades and a 1/4 foot. The ruler and rotary cutter, with practice makes every piece exactly the same dimensions. You have to work out exactly what side of the line you cut on and be consistent. Use a ruler to check your pieces until you know where to cut for the exact dimensions needed. Or mark the fabric and cut the same for each piece.

Once you have your pieces accurately cut sewing them together is much more accurate but the 1/4 inch foot is a game changer. It has a little bar on the side of the foot which rests against the two pieces of fabric when you sew them together so your seams are all exactly the same width.

I know all the YouTube sewers zip down a seam but I go fairly slowly (but not too slow) to make sure I sew accurately.
I hope this helps, best of luck.