late drop

Hi guys,
For some reason I tend to drop on the wave too late all the time... when the wave is flatter there's no problem and I can just stand up and drop smoothly but when it gets bigger and steeper I always end up falling from the top of the wave or just not taking it as I can see I'm gonna fall and crash...
Can anyone tell me what I'm doing wrong? is it because I'm not paddling enough? or fast enough? when do you know it's time to stand up?
Cheers
For some reason I tend to drop on the wave too late all the time... when the wave is flatter there's no problem and I can just stand up and drop smoothly but when it gets bigger and steeper I always end up falling from the top of the wave or just not taking it as I can see I'm gonna fall and crash...
Can anyone tell me what I'm doing wrong? is it because I'm not paddling enough? or fast enough? when do you know it's time to stand up?
Cheers