So something is causing the back pressure on the line to be greater than the PSI the pump can produce. What is max PSI for your pump? Do you have a PSI gauge installed? If not, that would be step one for me. Do you have a pop off valve installed? Speaking from personal experience on what has increased the back pressure on my lines - condensation. It came on slow and gradually eroded performance. Do you have any big dips in your weighted line? It's probably not that though, seems I am the only one that's ever had that problem.
I have done a dead-head pressure check and it will produce 41psi.
