What baits are you using glider?
It may not be the actual size of the hook that is the problem, but the pattern.
I catch a lot of Tench and carp using 18's hooks, but depending on the bait, the patterns vary quite a lot.
For instance, when pellet and corn fishing, I use a wide gape hook so that one the strike the hook is not being struck back up inside the pellet/corn, but outwards towards the lip of the fish.
Whereas when using maggots and castors I like to go for a narrower gape hook pattern so the bait masks the hook more, or in the castors case, the hook can be completely buried inside it.
I would advise you go into your local tackle-shop and have a word with the staff about your problem, and they would gladly recommend a selection of different hooks to try, to match your tactics/bait.