I have tried everything imaginable (Iphone restart app reinstalled, Bluetooth off / on), nothing helped. A battery exchange stations on the belt brought no connection. So I went on the internet to look for solutions. On a forum, it was reported that the transmitter may hang (for whatever reason) and then almost to be "restarted". For this purpose it is not enough to remove the battery and reinsert. Rather, the sender without the battery needs re-clipped to the belt and then around as possible. 60 sec belted, hence the last ounce of power is drawn from the transmitter. Then reinstall the battery and it should work again, at least it has for me.
The product would get in fully functional condition 5 Stars. Through the above-mentioned problems, which apparently occurs quite often, I have to deduct a star. Because you have to look awkward long for a solution or the manufacturer is offering a solution in eg Q & A's, there to deliver another star.