Just had this issue for a while on my 2010 Sport. Bought the OEM relearn tool off amazon for $35 and tried to relearn each sensor. Doing it this way, I found out it was only one sensor (front driver) that didn't respond and ended up buying a new OEM sensor off amazon (40-60), taking the wheel off and having a local shop replace it ($15).

I always recommend sticking with OEM - TPMS12

But the sensors operate at 315MHz, if you are to get aftermarket make sure that is the spec. Keep in mind not all aftermarket sensors play nice with the Ford TPM system.