This is usually made on one of two hand types:
A doubleton
An intermediate-strength balanced hand
Passing Out the 1 and 2-level
If the opponents have a fit, you probably do too. Furthermore, if they can only make seven tricks, surely you can make eight...
Bidding 4N as Keycard, Every Time
First, you need a suit established as trumps to ask for keycards. Second, not every slam requires bidding keycard.
Defending Too Actively
They lead. Every. Suit.
Not Preempting