You have to wait...it can take some time (perhaps as long as 24 hours) for the group information to fully sync between all group members. Once it settles down, it should be fine...but it will be sluggish when you first join...that's just how it is.
Occam's Razor nearly always applies when troubleshooting technology issues!
If anyone has been helpful to you, please show your appreciation by clicking the button inside of their post. Please click here and read, along with the threads to which it links, for helpful information to guide you as you proceed. I always recommend that you treat your BlackBerry like any other computing device, including using a regular backup schedule...click here for an article with instructions.