how can i get a sub-contractor to pay me for work done last yr

I did some work for a prime sub-contractor at Nellis Air Force Base, while doing the original work we found more damage in the fire chases and i was instructed by Ellard Comstock senor project manager who ran the las vegas office, to do the repairs, he even gave us a drawing of how he wanted us to do the repairs, i have a letter of intent from him and since he no longer works for Northcon I asked him if he would write a statemnt for me and he did. I also have daliy reports where it states that i was doing the work and no one ever stopped us from continuing the work. I was in constant contact with Ellard and the super on site. the job was put on hold back in April of 2019 due to mold and i was told in 2020 that the job would be shut down permantly and northcon would no longer be part of it and i was asked to rebid it but for the prime, after the mold was removed . i then put in for the extra work done in the fire chases and i was told NO, because there was no change order. NOTE: I did another job for them at the same base and i also had a hard time getting paid. ive gone to the contractors office and have been told that i am not the only one that has problems getting paid from them and that other contractors have complained about them. Thank You Olivia Carlos