If your account was on probation then maybe there is much Bitcoin on hold. When you sign in your account returns to normal status. Then all hold amounts sent with next distribution.

Bitcoin is also on hold if below a minimal amount. This amount is set on settings page. But there is also a minimal amount enforced by transaction server to avoid dust problems. This minimal amount can change depending on Bitcoin environment. Right now the non-dust minimal amount is 0.0005 BTC.

Minted seats have fixed minimal amount of 0.001 BTC.