taking an enema

I have no problem giving myself an enema, but I have a great problem getting more then a a cup or so of water inside me before I have to expel it. What am I doing wrong? Any help with this will be greatly appreciated as an enema this small does not really do much good. I continue to administer trying to finish a bag but get frustrated when I leak & don't seem to be able to increase the amount of fluid I can take or retain.