There are poor teachers. There are problems at home. There is little motivation.
We test them but we don't go over test with them. Why are old Iowa Test/ACT/SAT with answers and explanation not given to kids so they may learn what they did wrong? After all, the whole point of education is learn?
Here is the Visual Math Quiz of basic arithmetic. I may make it a series to cover math topics beyond basic arithmetic.
Why is it easier to learn the basic operations if you WATCH a teacher do it on the board? That's because the teacher can do it one step at a time. Sure, you can tell the teacher to do another example, but the teacher can only do so much. So, the calculator can do this, but it doesn't show you the steps.
How about you enter 2 numbers and select an operation and see it carry out the steps.
How about clearing up the confusion of integer arithmetic in number line demonstration. See, adding means count to the right, subtraction means count to the left.
Get yours here