First, it could be the cutting. Are you using your rotary cutter straight? Or do you lean a bit? This can make a difference. A couple threads per cut add up quickly in a block. Second, your stitching. Practice your 1/4" A LOT. You will be happy with the results. But, if you do have a block off a bit, a good thing to remember is 'we are bigger on the bottom'. This means to put the bigger piece on the bottom and the presser foot will help ease in a small amount.
Third (and most important), just keep quilting!!!! The more you quilt, the better you will be. Small errors are actually new design elements. Chances are that no one will notice but you. Quilters can be so hard on themselves, just have fun and keep quilting.

I have been trying for years to make a perfect quilt. Sorry to say I haven't made a perfect quilt yet.
I try 3 times to get blocks to line up after that I just go on.
Remember we are our worse critic. We see quilts up close, on the bed it is harder to see.
Just keep going and you will get better.
Welcome to this board, lots of help here.

Square up your blocks and if you can, square up the pieces that make up your block!!! I do not have a lot of issues with getting my blocks and then my quilt to come out right because I square up...a lot. It is definitely more time-consuming to go this route but it pays of in the long run :)

Such a good idea about the 1/8 in extra- I learned accuracy by working on a log cabin and it was a combo of cutting, 1/4 in seam and using the Shape Cut Ruler inconsistently( cutting angles in stead of straight)
It was frustrating to say the least but I learned so much about how to measure after each seam sewn, accurate width of strips and improving my cutting. And the illusive 1/4 in seam. Measuring as you sew helps your accuracy. I use a 1/4 in foot without the guard- works for me- best wishes

I have been quilting since 1992 and I have yet to achieve the perfect block. My 12 1/2 blocks always seem to end up 12 1/4. I have stopped worrying about it. As long as I am making all the blocks in my quilts, they will be off by the same amount.

You could unsew once, maybe twice but I don't think it really matters if you are not doing intricate blocks. I have found that if I get really picky about my blocks I have more trouble matching seams. If I relax & do my best a quilt can go together easier & faster & look very good. I'm not saying bee sloppy, just relax & do your best.

Pressing sometimes affects block size. The crosswise grain of the fabric has more give to it (and will stretch slightly when pressed) than the lengthwise grain.
When sewing two blocks together, place the slightly shorter one on top. It is easier to ease in excess on bottom block.

Do not rip it out again. It's too much work! Really examine some of the quilts you see in magazines and you will see that many of them have seams that don't quite match. If the magazines can't make everything match for a quilt that they display to quilters all over the world, then you can forgive yourself if some of the blocks don't quite match.

Just keep trying to improve; eventually you will be more perfect. But no one has ever made a 100% perfect quilt!

Besides, the problem could be a cutting problem. You can't undo a cutting problem unless you have enough fabric left over to re-cut the quilt or even parts of the quilt, again.

One last thought: Sometimes the blocks are fine, but matching the seams where they intersect is off by a bit. We have all gone through that, and, sometimes, in spite of our best efforts, the seams simply won't match no matter what technique you use to match them.

Before you sew the blocks together, try using some elmers washable glue sticks to hold the matching seams together before you sew the blocks together. The seams should nest together. The glue makes it harder for the matched seams to move when you are sewing them together. If your seams don't match by 1/8th of an inch, this method will help you ease (slightly stretch) the extra 1/8th inch into alignment.

Pressing is very important , I found that you do not push the iron but pick up and press down. You can stretch the fabric by pushing to hard.