If you have 2 or more Oregon sensors of the same type and with the same channel selected you will have sometimes the problem that one or two sensors are not received for a period of a few hours. The reason is that they transmit during time on the same moment. The interval time differs a very little bit and the transmit period shifts slowly and at a certain time both sensors transmit on a different time again and are received then again.
The solution is to select another channel on each sensor of the same type or use different sensor types. TH1, TH2 are different types but THR128 and THC138 are the same types as they are both TH1 sensors! There are also 10 channel sensors for example the THGR810.
It can still happen that sensors transmit at the same time but not for a long period then.
See also this forum message. http://board.homeseer.com/showthread.php?t=130500
Multiple Sensors
Moderator: Mattk
Multiple Sensors
I received this mail from Bert at RFXCOM, that might be helpful when using a lot of Oregon sensors: