Spn 157 fmi 18

I currently own a 2012 Volvo Vnl with a Cummins isx, a little over a month ago a code popped up (spn 157 fmi 18). I took time and looked the code up and it read low pressure in fuel rail, also I must add the truck was running and starting just fine. About a week went by, the truck didn’t want to start, when we got the truck started it was missing and had a loss of power. I took the truck to the shop, they changed both fuel filters and the actuator on the fuel pump. The truck still wouldn’t start but when we got it started it ran much better. I then took it to another shop, they changed the a valve on the fuel rail and a leaking fuel line but still same results...no start. They recommend to take the injectors out to get tested, so I did. 5 injectors came back bad so I just went on ahead and purchased all 6, they replaced the injectors and once again still no start
So I did some more research myself and found out for that specific code (spn 157 fmi 18) it can be the fuel pump. So I had a mechanic take the head of the fuel pump off to make sure there wasn’t any ceramic taplets in the pump head but when he did we came to find out that they where already updated to the medal Taplets. This truck has seen several shops for this one problem and I can use any advice I can get. Thanks