Just jumping on this as I had the same issue and it is now fixed.
(edit: this is a NON Power Latching Door. Just standard mechanical latch)
T6 rear sliding door refusing to close and latch. It was just bouncing back open no matter how slow or fast I closed it.
It turned out to be a snag in the cable connecting the handle mechanism at the front of the door with the latch mechanism at the rear.
I had ordered a replacement latch after reading some of the comments that it is likley to develop a fault, and only when I was in the process of installing this did I discover that there was no issue with the original latch, other than something was stopping the cable from returning to its normal rest position. It was sticking around half way (creating slack in the handle too so look out for that as a sign).
Once I unbolted the latch from the door itself and pressed the lock mechanism (silver part connected to the cable) it returned to its normal home position and the latch stayed in the locked position.
I reinstalled all the original parts and the door now closes and latches perfectly again.
I hope this helps.
(Photo shows the Silver return mechanism which in in the correct position in the photo. It was stopping half way through its stroke and that stopped the latch from securing the door, which resulted in the door bouncing back open every time.)