How are you checking your stones for flatness?

What are you using to flatten your stones?

If you are going to use a stone for razors and sharpening other tools like knives and chisels, mark the stone and only use one side for razors. Flattening a stone is a process, involving gridding a stone and lapping it flat multiple times.

If you grid your stone and the grid is not removed in 10 laps or less, the stone is not flat. If you hone straight strokes on a crowned stone, you will create a frown.

You are creating a frown because your stone is not flat, or you are not using an X stroke. Probably why you have issues at the heel and toe.

You will have to remove the frown, by bread knifing or high angle grinding the edge flat and parallel with the spine. Then hone the razor to get the bevels to meet again.

Here is a post on lapping a stone flat, it is not a grid once and quick lap and it is flat job. (Synthetic Stone Lapping 101)