trans problem , not initially engaging in reverse when cold .

not sure if this is related to the engine mounts / trans mounts i am about to fix , because they are all bad but , when i start the car ( 1974 valiant /6 auto ) in the morning , and put the gear selector on R nothing happens . if i then go to D it usually engages , then i can go back to R and then it works . or sometimes if i just rev the car a bit in R the first time , it will finally grab . i just got this car and have not done anything yet . but this one worries me . is it the mounts ? or something simple like a clogged filter / gear selector ? could i get that lucky ? or is it something more serious ? thanks