Time management seems to be the big one. I only have 2 partners but there are some definite issues with making the time to see them.
In between me looking for work, going to school, volunteering, and making time for my good friends... it's tough to make sure I have enough face to face time with my partners. Then add in that my romantic partner is even busier then I am and is in 2 other relationships... And then my FWB is almost always working, with his kids, or with one of his other sweeties...
I get to see my romantic partner maybe twice a week at the VERY most. I get to see my FWB once a week but sometimes I go 2 weeks without seeing him.