TPMS Problem(s)

2007 crew 1500 Sierra with a TPMS in the DIC. It turned cooler about 2 weeks ago, and upon start up, the DIC said RightFront tire pressure problem. Checked and it was about 2-3 psi lower than the set point of 35. Normal for a cold morning. Next day, the DIC told me both right side tires were low on pressure. Put 40 psi all around, did a tire learn and seemed normal. This morning, temp outside about 70, DIC said check TPMS system. Tried to read Tpsi, all had - - instead of a reading. Did another "learn" 3 of the 4 had readings, one LF had - -. So either all 4 sensors are acting up or the TPMS receiver is flakey?? How do I troubleshoot this? Had all 4 tires replaced about 6 months ago, TPMS sensors were "serviced/rebuilt"