It comes down to flexibility - how flexible your tendons and ligamints are. Some people have stiff tendons that do not take to kneeling well, not for long periods. Others have more flexible ones. Kneeling is not really supposed to be an uncomfortable exercise, however (in fact, when subs complain of pain during the tasking soc kneeling tasks I usually tell them that the goal is not the pain and they should find a way to kneel without discomfort). Usually we kneel because it is a submissive position and sometimes it is because it is a position that presents certain parts of your body for access or display (especially some of the gorean positions - at least one of them is designed to present the breasts).

Rice is VERY painful. I warn you now so you know... I have also seen tasks where the sub is expected to kneel on stone or concrete (equally nasty) or in something horrible like dog poo. Sometimes discomfort can be created by the position of the arms - for example, holding them out to the side with something balanced in the hands or held behind the back in a 'tied' position. Same applies to kneeling against a wall with a coin pressed to the wall with your nose.

Basically, I would be grateful that you can kneel comfortably (unlike many) and hope like hell a Domme never tells you to do any of the above or that you never meet Den's old Domme