If these roaming SIMs are GSM then as a start check out SIM EFs in standard GSM 11.11. Understand content of those EFs.
Check your extensive data, assuming you have data, to know about handsets reading low RxLev and check parameters for MS to ‘camp on a cell’. Check idle-mode MS cell lists returned to netwrok on SDCCH. If low RxLEV detected low during calls what are the power-steps to ramp up power detection?
I guess at answers because there is not much detail in your question about how you know problem with RxLev in first place, which mobiles are relevant, and what tests you have run?