First thing I would do is try making the same cut with some scrap pieces of wood.
Be sure to cut a similar sized piece of wood. (Example is 1&1/2" wide piece for your test cut if you are having problems with cutting blocks that were 1&1/2" wide)
Also use a hardwood like maple or walnut.
This should tell you if something is out of whack with your saw.
I always cut a test piece first before gambling with the good stuff.
By doing a test piece I am able to insure that my table has not been bumped out of square.
I also use a test cut to double check my thickness of the cut I am making.
If you get the same results with the test cuts that has been happening to you it can be a variety of different things.
If the blade is not tight enough it can flex or wander.
Your blade guides might not be set right. I use a business card to set the space between the blade and the guide.
The top guard should be lowered to a point just above the height of the piece you are cutting.
If these adjustments don't correct the problem, next thing to consider is your blade.
If you hit something with your blade like a rock, dirt or a piece of metal it can chip the side of some of the teeth. In addition to dulling the blade it can also cause it to wander.
If you have been cutting and the blade got bound or stuck in the wood it can become distorted.
Sometimes it will just be bad blade(s)
Crummy weld, cheap steel or bad cutting of the teeth.
Blades are like knives, If you find cheap ones there is probably a good reason.
I like Lenox Bi-metal blades myself. They cost about 3X a normal blade.
But if it keeps you from messing up just one block it has paid for itself.
If you check the blade and all the adjustments and you get good cuts on the test pieces but bad on the actual blocks then it narrows things down to 2 things.
#1 The wood
If it is natural wood that was not kiln dried correctly, the wood can experience case hardening. This causes the wood to spring into another shape when cut.
If you push too hard and try to cut too fast your cut will start out closer to the fence and then gradually wander outward. If you push the block too hard or at an angle with your push stick the blade will wander back toward the fence at the end of your cut.
If you use slow and steady pressure when making your cut, the blade will determine how fast to cut and you should get an even cut even with wood that varies in hardness.
Hopefully this will help.
Don't ask how I learned these things.
One last piece of advice.
If you go to a local cabinet shop they will usually be happy to give you a box of junk, cutoffs.
Cut these into blocks about the same size as the pieces you will be cutting and keep these next to your saw.
Every time you will be cutting scales, use one of these pieces and make a test cut.
Then you will be able to make sure the saw is cutting correctly as well as double check the dimensions of your cut.
There have been times I thought I set my cut for 3/8" thick and had mis-read my tape.