Ender Dragon doesn't spawn gateway after Death

Hi, 

I killed the Ender Dragon twice, once in survival and once in creative after respawning him wih ender crystals.
None of the times it spawned a gateway (the 1 block portal to go further in the End). I should actually have 2 opened now.
I didn't see any reports about this matter so I hope not to disturb for nothing. 

Thanks in advance, 

Mutzhag