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Hi, with limited amount of information I would say have a look at the K factor of the inverter's electrical control (i.e. Kqv). You have to be careful though because normally this value is set to 2, if a grid is not strong enough (lack of reactive power capabilty) then a higher value might result in system instabilities.

You would also have to check the trigger voltages, namely Vdip and Vup; as well as the deadband for voltage control db1 and db2. Normally there parameters are set to match, for example if you want the inverter to enter fault ride through mode at a +-10% deviation, then you will set Vdip and Vup to 0.9 and 1.1 respectively, and db1 and db2 to -0.1 and 0.1.

If you are still not getting the desired results, I would say check the reactive and active current signals, check that the remaining control portion of both the electrical control (REECA) and the plant controller (REPCA) are inactive or frozen (for that you also would have to check Vfrz in the REPCA), and finally check the states and calculate what is going on at the FRT portion of the electrical control.

I hope that helps!