4G always turn to H+ after a certain time

4G LTE
1

Dears, good day.
In our network 4G always turn to H+ after a certain time, and we turn the flight mode on and off to return to 4G.
Any one have any idea?
Vendor is Huawei

2

This is inter-RAT handover from LTE to UMTS, you need to check 4G RSRP or B1 threshold, H+ is dedicated mode in UMTS with 64QAM or DC-HSDPA active, can not handover in dedicated from 3G to 4G.