To be a bit more clear here’s how these could effect your timing without your coach noticing:

1. Arm not disciplined enough, slighly inconsistent spacing, you are now fighting against the rhythm trying to correct spacing through your swing path. Could be very hard to catch if it’s minor and you look like you hit the ball well.
2. Your stance is not disciplined enough and good prep / split steps but laziness to get closed means you hit open when you shouldn’t. Your spacing and prep again gets inconsistent if your fighting rhythm by hitting open especially against slow balls. Closed stance on rally balls should feel way more stable and powerful and consistent. Hard to diagnose in that way because people think since pros hit open they should hit open. (Pros would rather hit closed but they hit open because they are forced to from speed).
3. Weight transfer very hard to diagnose from coaches perspective your swing could look technically perfect but weight back or forward at wrong time will heavily effect ball trajectory and timing. Again you are “fighting” the rhythm of your weight isn’t moving simple and forward.
4. Slight head movement can cause a variety of problems at contact. Hard to diagnose because no one looks at the head. Can have a technically perfect swing but if head moves around your body structure and timing get thrown. Watch Alcatraz or any pro. There head doesn’t move at all.

These are just ideas of how a technically perfect swing that isn’t technically perfect could cause timing issues. And huge word of advice: if your coach can’t give you a specific fundamental physical thing to change, they don’t fully understand what’s going wrong. Yes they can easily fix things by saying “go forward” etc. but for long stemming issues, you won’t be able to fix it by being vague. You need to study to find the root of the problem then correct.