I don't use the automatic fstrim, I do it manually.
To clean junk I use the terminal every now and then and enter the following,
e.g:
for root partition: sudo fstrim -v /
for home partition: sudo fstrim -v /home
You can adapt this for any other partitions, e.g. data partitions..
I picked this up from Quidsup on Youtube.
Regards
Mike