Have them replace the valve.
Recently picked up a 2008 crew cab. On my initial test drive, the TPMS was warning that one of the tires was low, so I informed the dealer that I would require that to be looked at before I purchased. He confirmed that the TPMS was working, one tire was low so I worked 4 new LT tires into the deal. After having the truck home for a few days, the low light indicator came on, and sure enough the same location was down again. I pumped it up, and kept checking it. Few days later again it was down, so I made an appointment to have it checked. Once again the dealer mechanic inspects it and tells me he re-mounted the tire, and everything should be OK. However, it is still loosing pressure. about 1 PSI a day. I guess its back to the dealer again...
Any suggestions as to why this is occurring?