I think by default MC sets a limit of 3 episodes. What it should do when the 4th comes along is delete the oldest and record the new one. You can adjust the number to save under Series Info.
I should also mention I have had occasion where the series settings got corrupted. I had it set to record and save 10, but it was deleting episodes at 3. I had to delete the series and create a new one. All was good again.