My DD only poops about once a week, sometimes twice. Now, mind you she never had slow weight gain, But both midwife and pedi said that as long as weight is on track and she isn't going more than 8days or acting like she's in pain that long intervals between poops was okay. I think the weight gain would concern me tho.
As for not being able to get anything out with the pump, have you tried hand expressing? I know, I know it's a pain, it takes longer, etc....but I was the same way, and I can tell you it does work. (DD was in the NICU for 2 1/2 wks and was unable to eat by mouth for 1 1/2 of them, I managed to get my supply started with hand expression). and like several PP said, you can supplement EBM with something like a lact-aid or SNS which will help bring up your supply. Also agree about having LO's suck/latch evaluated.