Transmission...going?

Sometimes when I try to drive in reverse the car doesnt go backwards it just makes a sound like its trying to engage the clutch but it can't. This only happens in reverse, 1-6 work great, and it seems to only happen when the car has been sitting for a while...so..uh...I'm gonna check the fluid make sure it isnt low or somethin. Think this is a fluid thing or is my reverse slipping?

fluid first then possibly clutch is going out or possible synchros

definately check the fluid first