Turning over but not cranking every time

"Turning over" and "cranking" are the same thing. It means the starter is spinning the engine. You said sometimes when cranking, it is not cranking. Is it or is it not? There is a common problem that causes the starter to not crank the engine, and that always starts out as an intermittent problem. There is some different problems where the starter cranks the engine just fine, but the engine does not run. For that you will need to list the engine size. There are multiple engines with very different ignition systems, and they have different diagnostic steps. If this is what you have, please list as many clues and observations as possible. Most of the time heat is one of the variables that affects when the problem occurs, and that is related to stopping a hot engine for a short period of time. If you can identify anything to suggest when the problem will act up, include those details too.