First you need actually provide us with some logs from raids. Several fights is optimal so one can rule out fight specific mechanics/heal comp/having a bad night (we all have them) etc. Also easier to spot patterns with multiple logs.
If your guild doesn’t log you can log yourself as well. Warcraftlogs is preferred, found here: https://www.warcraftlogs.com/
Create a user for yourself, download the client (this is what allows you to upload logs to the site), make sure advanced combat logging is toggled in your system – network settings in WoW (this allows us to look at talent choices/gear etc.). To activate combat logging in game type /combatlog the same command is used to turn logging off. You can choose to live parse or just upload once your raid is done (this will mostly depend on your computer). For an excellent guide to logging check out Tuskeh’s youtube video – click me.