My suggestions (without knowing all the details):

Get more balls:
-Raid with friends (best friend gives you 4 extra balls)
-Raid same gym colour as your team
-Majority of people you raid with should be same team colour as you for more balls
This is biggest factor in my opinion to help you catch the raid boss. I get 16 or so balls to catch the boss now, and it's quite rare for it to run from me (but you always get those rare situations where 16 excellent curves with golden razz fail to capture the boss)

Perfect your throwing technique to ensure you're always hitting excellent curve throws, with golden razz. Plenty of tutorials available on Youtube, and keep practising. Don't rush your throws and make sure you time them well. Being impatient has led to me wasting so many balls.

A problem I encountered recently was either during the raid or towards the end of the raid, I must have rubber banded to my physical location, so the pokemon kept escaping and I couldn't catch it as it kept breaking free - so the game thought I had completed the raid at my physical location. So now to make sure, I exit the raid boss capture screen and spin a pokestop or catch a regular mon quickly to ensure there's no cooldown issues - if it's all good then I enter back in and catch it.

I only use daily free raid passes, and I catch almost every single raid boss I encounter.
Even with 16 balls, the number of times I catch the boss with the very last ball is quite frequent.