Bad connectivity issues with a few DA14531MOD

We use DA14531 modules in our production units. These modules are pre-programmed and tested before adding them to the main board. 

We have noticed that a few production units suffer from bad connectivity. At closer ranges (less than 2 metres), the RSSI value drops to about -85dBm, which further drops to about -100dBm as the receiver is moved further. Some have a reduced range of about 4 to 6m. A few disconnects within a couple of metres range and then does not advertise again. The user can see the advertisement name again once the unit is rebooted.

These modules were all tested while running at the highest power level (AT+PWRLVL=12). 

Is there a way to test and verify what is causing the bad connectivity (Any hardware failure, antenna issues or insufficient power supply consumption). If not, would it be possible to test any particular value and reject the module before adding them to the production boards. Most of the time, RSSI does not provide much significant data at closer distances (before adding them to the board) during production stages.

P.S. The Codeless version has been upgraded from SDK6.380.12.6 to SDK6.380.16.55, which did not solve the issue.

Kind Regards,

Arun