I've found this typically with swat vehicles. I have 9 swat trucks, and Sometines when attempt to send 4 to an active shooter or any swat call It says I have no swat trucks available. Even if I have no pending calls and I know all my vehicles are available for the call.
Here's the bug, it says "vehicle display limited! Load missing!"
I press load missing, and my 9 missing trucks load in.
Yes I know the solution is right there and I'm super lazy.