OpenGl ES + iOS, why is nothing drawn?

Question

iOS

I am developing an iOS application using OpenGL ES. There are 3 GLKViewControllers that I switch between as I work. Each of them draws a set of objects (lines, polygons). Everything works fine, but sometimes when the next GLKViewController is loaded, nothing is drawn on the screen. All that remains is the color fill, but there are no lines or polygons.
I checked everything I could think of. The draw function is called, the camera is not moved anywhere, nothing has changed, but there is no image.
The error appears randomly. Sometimes you need to switch between controllers 11 times, sometimes 35.
Also, if you load another controller (or reload the same one), the problem disappears.
The problem appears only when loading. That is, if the download went well, then nothing will happen in the process. And if it didn't go well, then you can't work.
I have no idea what could be causing this and where to dig to solve the problem.
I hope for your help.