Is it higher IV' mon is more difficult to catch??

IV does not influence catch or flee rates

I have only ever not caught one snorlax according to my dex. It was a 95% I know because someone raiding with me had the same cp and caught it. My highest currently is an 84%. I think it's all rng but I feel your pain!

Catch rate is determined by species, level, and throw bonuses. IV does not factor in. If your trainer level is 20+, all raid bosses will be L20.

Well let's say you've got a CP640 Dratini in front of you. If his IV is low, he'll be a higher-level Dratini. If his IV is high, he'll be a lower level Dratini. So technically speaking, the higher-IV (and hence lower level) Dratini should be easier to catch.

IVs have no effect on catch rate.

https://pokemongo.gamepress.gg/catchcalc#/
The variables for catch rate are:
- Pokemon species
- Pokemon level (always 20 for raid)
- Pokeball (same as a red pokeball for raid)
- Razzberry or golden razzberry
- Curve throw bonus
- Hit bonus
- Catch medal bonus

The same pokemon at the same level have the same base catch rate.
In fact, if there were two wild pokemon with similar CP but wildly different IV%, the higher IV% could be a lower level and thus a little easier to catch.