I've had too Blackburn Airsticks, the first one broke eventually after a lot of problems with it and as they are guaranteed for life I got a replacement but I found that they are useless. The pump is designed to inflate on the forward and backward stroke, my first pump I discovered only inflated on the forward stroke so it was hard work with it. When I stripped it down, their are a couple of tiny valves held in place with just as tiny springs and one of them had come unseated, the pump worked for a short time and stopped working all together eventually. The other thing I find with them is they fill with water easily which emulsifies the grease on the piston rubber seal and you loose pressure, a quick strip down, tip the water out and dry the cyclinder and regrease can improve things. I also fitted a couple of inches off a 23mm inner tube on the pumps body to cover the gap between the body and the handle to help prevent water getting in and also to stop the handle coming out of the pump as I rode as I found it best to carry it handle downwards, once again to stop water ingressing.
Put your finger over the pump and see if you get pressure on both forward and backwards strokes, if not then one of the valves is misplaced so it will need a strip down. And as you say it could be the wrong pump for a MTB type tyre, they do need a lot of "wind" in them, but if you could only get to 55lb, then I suspect, the piston on your pump needs a grease as you cannot build the pressure up to open the valve in the innertube to force air in and instead the air is forced past the seals inside the pump to take the easy route.
My ultimate solution to solve the problem as I deemed them unreliable, was buy a couple of Leyzne HP pumps and never looked back.