My truck does the same thing, so what I do (instead of starting and stopping the engine) I let the clutch out, then push it back in, and after that, it drops right into reverse. Sometimes, the teeth in the gear box don't align just right, so letting out the clutch and letting the input shaft spin briefly, and pushing the clutch back in will do the trick. If their is a deeper explanation for this, let me know, I'm willing to be corrected and learn. Isn't that what this site is for?