Android not roaming when in sleep mode on Smart Zone

unknown

We have SmartZone controller with nearly 200 R500 APs. We are finding that android devices (Samsung J3) do not roam to a new AP once in sleep mode. We have a Mitel SIP client App that works fine in sleep mode if you stay on the same AP, however if you let the phone go to sleep mode and move to another AP it will njot accept any calls to the app until you unlock the phone. The phones are on a dedicated SSID with QoS applied. Any ideas what could be the cause?