nt765b starting problem

ok so our challenger has an issue that we are having problems with the c9 motor getting started if its sat for more than a day. we have had holt cat out on 4 or five different times and they have replace the check valve on the head twice, and the pump. we have also changed the oil to a lighter oil and that helped starting when its cold but only if its started daily. now if its sits for more than a day then it cranks and cranks but wont catch. even if you either it taday it took 5 different either times to get it to run. so its like it has no fuel. anyone have any insight to why a c9 cat is leaking fuel back so it wont start or if there is something else that you know of to check.. all voltages come up right also..

There is a check valve in the fuel base and a non serviceable one in the transfer pump. Only ones I know of. Leaking lines or maybe orings on injectors.