The tense is wrong. You're not slipping. You've slipped.
5 seconds sound clip from the A Star is Born classic radio play.
You can hear this line at 00:30:34 in the radio play.
Quote context
[...]
- Mmm... Er, let's wait and talk business at the office, Norman, hm?
- Well, maybe the part wasn't just right.
- It was the best part of the year.
- Now, look here. Do you...? Do you think I'm slipping?
- The tense is wrong. You're not slipping. You've slipped.
- Well, my fan mail's still pretty big.
- But every quarter they pay for a theater ticket buys them the right to be a critic.
- And your last few performances, Norman, haven't pleased your critics.
[...]