It is very hard to prime a long suction pipe.
It is even harder if the suction pipe is not RISING slightly, with no high spots or humps, from the suction end to the pump.
If there are any humps (high point followed by a lower point) between the inlet and the pump, then air will be trapped at the high point and the pump will not prime.
The only way to fix it is to get all of the air out. If you can, try applying a wet/dry shop vacuum, through a valve, to the line at the pump end until you get a good flow of water. Then turn off the valve to keep from losing the prime.
Then start the pump.
If you have a hump and you know where it is, you can fix it. One way is to run the pipe on a rising grade. Another way is to put a tee in the line at the high spot, with a ball valve, so you can prime the pump and vent the air at the high spot. I would resort to that only if you can't get it to prime with the shop-vac.