There are specific maintenance procedures to do with slide seals. Involving cleaning and lubing the rubbers, cleaning the RV wall, removing debris from on top of the slide, checking the mechanism for 'drift', that sort of thing.
Also, many users eventually buy those plastic kids noodles and put them under the slide awnings to help shed rain better. Keeps it from puddling on top of the awning.
Suggest a couple hours perusing the owner's manual.
Also, please put the age, brand, model, and engine type of your RV in your signature so we know what we're dealing with. Answers for a 1 year old RV are quite different from a 20 year old.