With sweet cream I get about 2 - 2 1/2 lbs of butter per gallon of cream, but with culturing it goes to 2 1/2 to 3 lbs of butter.
I'm sorry, but I don't know your details. Do you have cow(s) or where are you getting the cream? I have found that I need to refrigerate my milk for at least 24 hours before skimming the cream off or I end up getting a lot of milk in with the cream, which really reduces the amount of butter, too.
Also, what does the butter look like when you quit running the mixer? It should be forming clumps. If it is just grains of butter, you are losing a lot of cream yet when you strain it out.