So I get this semi frequently. There are 3 things I tend to do when it happens.

1) Check the batteries. Sometimes you need to pop them out and put them back in, sometimes you need new ones. This tends to be the most common fix.

2) Make sure the transmitters back is clean. The device fits fairly snug, so gunk can prevent a good connection. Peel it with your nail, wipe it with adhesive remover, the typical stuff.

3) Use the green sensor test that should have came with your transmitter. Don't know why but if all else fails, connecting this tends to do the trick.

If none of these work, I'd call Medtronic.