Check these possible causes :

1. One of the headsets is out of range. The quality of the Bluetooth connection (and associated audio distortions) depends on the weakest connection of the two connected headsets. For example, audio distortions may be heard by the wearer of the headset closest to the HearLink/HearLink PLUS transmitter when the second headset is out of the transmitter’s range.
2. One of the headsets is not a BeHear headset. The active CODEC (SBC, aptX or aptX-LL) for the HearLink PLUS transmitter is determined according to the common denominator for the both headsets. Two BeHear headsets will always operate in aptX-LL mode. However, if one of the connected headsets is not a BeHear headset, and it doesn’t support aptX, both headsets will work in the lower quality SBC mode only.
3. Each headset is paired to a different mobile phone. It is not recommended to connect two headsets simultaneously to one HearLink PLUS and maintain separate smartphone connections for each headset. This is due to problems which may arise when the wearer desires to switch the audio stream between his/her phone and HearLink PLUS (e.g., for an incoming call from one of the phones).