I've seen some people have trouble with the phonecall to Dimitri on Russian Revolution, as well as being a bit slow in the warehouse or getting health before.

The phonecall seems a bit strange, but you can get it consistently if you wait for the popup that says "You can phone Dimitri.." and then be at the right spot when the phone connects.

In the warehouse you can spam Forward and Jump to immediately jump to the armor. You shouldn't get shot before you get it. Then you can make your way to the health while shooting the enemies on the ground floor on the right/center. LJ should follow you if you do it right.

I made a video about that as well as added the info to the wiki: https://kb.speeddemosarchive.com/Grand_Theft_Auto_IV/Missions

ZreknarF