I thought I would raise this again as it starting recurring in the last couple of days.
I have about 3 different BT HRM's (a Polar and two Wahoo Tickr belts). For some time, they go through being uncooperative in connecting. I've tried removing the battery in the HRM to reset as well as replacing batteries. Not sure it helps. There is just some kind of bug in TDA, which I have reported many times. I rarely have trouble getting the trainer to connect, just the HRM's. Eventually, after restarting TDA a number of times and re-initiating searches, it finally connects. TDA always detects the presence of the HRM, it just won't connect sometimes.
Be nice if Tacx Garmin put some energy into this rather than cosmetic changes to the TDA.