My softclose function also gets stroppy on occasions and doesn't always pull the tailgate down - I have found that it usually happens after somebody else has driven the car and they have tried to slam the tailgate closed.
It always resets though.

Mercury check that the upper section is fully closed (even if it looks closed open it and reclose it)) - if it hasn't fully closed then the interlock will be operating preventing the main tailgate from closing. I once found that the LR rubber boot liner mat had moved and was trapped in the tailgate which was causing the tailgate to be on the latch but not fully closed. 2020 Pangea Green 1st Edition D240 New Defender 110 is here and loving it
2018 Melting Silver Mini Countryman PHEV - soon to be replaced
2015MY Corris Grey SDv6 HSE Dynamic, the best car I have ever owned, totally reliable only a cou0le of rattles in 3 years, now no longer in my care
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW