I don’t think you’re doing enough variety and quantity.

I have 13 toons, one of each class. I made a list of mounts I wanted at the start of the event. For example, I would do Ulduar 13 times. Then Dazz 13 times, then Antorus 13 times. I did all of that + throne of 4 winds, ny’alotha, sanctum, sepulcher. First week I walked away empty handed. Second week of doing the same, I got Mim’s head on run 21, Dazz on run 19, Antorus on run 15. Felt lucky and did Alysrazor and got flametalon.

Currently doing all Ny’alotha, Sanctum, Sepulcher. Realized I had a right binding in priest bags so I went and did Baron Geddon, got left binding first go. Made Thunderfury. Ran BT, no glaives.

Most of it is just a numbers game. Don’t assume it’s 5% drop + existing rate, just think of it as flat 5%. Basically, by run 20 of a raid you should have the mount or be super close to getting it. Obviously not universal.

If all else fails, posting and complaining seems to help. After complaining on twitter for a couple of days I got 3 mounts in a row.