This can occur for any of the following reasons:
1. Bathroom stall is already occupied
2. Adjacent stall is occupied (only applies to shy B.M'rs)
3. Toilet is dirty
4. Bathroom being cleaned
5. Toilet paper empty
Brownballing CAN occur mid-session. For instance, if you're mid-way through a loaf, and someone walks into the adjacent stall and forces you to halt operations...you got Brownballed.
Note: this does not apply to SELF denial of toilet services (ie. constapation, phone calls, etc.)