Colonoscopy had to be abandoned as prep didn't work properly. I will be having another one but how can I improve?

I went for colonoscopy jan 2025. Had to be abandoned 

Prep didn't work properly. Pain was too much. Ive been told ill get a letter for about 6 weeks time to try again.

I dont know what went wrong, how do I improve it?

Surgeon said I was constipated. Next time he's asked me to try sennakot.