Cory catfish keep dying?

Hoping someone can help! I have had a 300ltr tank running now for almost a year with no issues tank consists of types of tetra oto catfish, shrimp and harlequin rasbora. All parameters are brilliant including KH as I'm in a soft water area. Temp is 25.5 and it has a 20% water change weekly with sand vac.

I added 6 black cories and 6 bronze to my tank back in June and only have 2 blacks left and they have got a decent size and look healthy. I bought some peppered cories about 2 months ago and will randomly lose a small one now I have two die within 24hrs. Checked all parameters again and it's still all good. Am I doing something wrong? Why can't I keep cories alive. Also to add, in this period of losing fish over the year it has only been cories that die