Pumping up to the Ceiling

I'm in a building with no drains. A sink was installed with a drain pump which pumps the water 20 feet to the ceiling then a couple of hundred yards. This was done before I came into the picture. I have never seen it work. The company who installed it came and looked at it (warranty had expired) and said they would replace the pump and they would send a quote. They avoided us and never provided a quote. Maybe they saw a design flaw. I took it loose and some parts fell from the check valve/backflow preventer. I replaced that, the pump will work once then it doesn't seem to open the preventer. It seems the pipe is filling with water then preventing the gate from opening. We have another pump which pumps the same height but not as far.

stupid question, but have you checked the arrow on the check valve? it should be pointed in the direction of the flow

away from the sink, also, ck the direction the pump is pumping,,away fro the sink

also...what kind of check valve? spring check or swing check. a swing can only be installed on a horizontial line. it MUST be level
a spring, can be installed vert. or hor.

swing check has a gate that swings bac an fro

spring ck......spring /diaphram

Is there a valve after the check valve?
It would be a good idea if you have a 20 ft head. all that water will come back down if you remove the check to service it.

Another thing is to look at the pump curve. Some of those little pumps can't push water up verticallly 20'.