It's likely that the pump diaphragm has become stretched and wrinkled. If it's a pump that can be serviced, you can get kits to replace all the inner parts. If you leave the hoses attached and unbolt the pump from the block you can manually use the lever to work it. It may seem fine but the movement you do by hand will be much more than that made by the cam. The cam movement won't quite give enough push to the diaphragm to suck the fuel uphill. If you leave it, it will probably stall out if you get stuck in traffic going up a hill and you have to sit there idling. I think you can tell I've had this problem. I just fitted a new pump, it wasn't a Volvo.
Swap the pump over from your other Amazon to confirm that it's the pump. Check the "bad" one on the other car too if you like.
Some "pattern" pumps give more pressure than you need and cause carb problems so choose carefully.