The RFD2000 sled may not be connected at times when you are trying to do the RFID reads. This can happen if the device falls asleep while TracerPlus is running, causing the reader to not be re-initialized by TracerPlus.
If you exit TracerPlus and come back in, it should start reading again.
As a best practice, we recommend exiting TracerPlus and coming back in if the device has been asleep. We are still looking into ways to improve the re-initialization of the connection to the sled in such scenarios.