I’ve literally been on this for 3 days, and it’s been driving me crazy, but i think i’ve nailed it down
Cat > Sub Cat > Tag Group
Restaurants > London > Chelsea,Wembley,Wimbledon etc
Cinemas > London > Chelsea,Wembley,Wimbledon etc
If a sub category has the same name and tag group as one from another category, topics don’t appear in one of the categories when you filter.
Sorry about this, i appreciate tags must be a whole lot of fun for you guys.
Yes tags add so much FUN to our queries… 
I think I reproduced the problem, or maybe a related problem. In my repro, the categories do not have any tag groups assigned to them, so tags can be used anywhere.
A topic tagged with many tags belonging to a group doesn’t show up when filtered by category, subcategory, and one of its tags.
The subcategory name and tag group name are the same, but I doubt that’s a problem…
certainly seems similar, the reason i thought it was because they had the same name is because the posts showed when i changed the slug on one of them to ‘cambridgeshire1’
what do you get if you change the slug?
right now, changing the slug leads to 500 Internal Server Error, and it messes up even more 
FYI - If the subcat name and tag group used are the same as another category
It wasn’t the tag group name, it was the identical subcategory names. Subcategories with the same name can mess up the tag filtering. I committed a fix for it. @GSeven Pull the latest commits and see if that fixes the weird behaviours you’re seeing.
Sanity restored!
Well done mate, that fix works a treat, thanks for all your time and effort.
