Volvo s60 t5 auto gearbox problems

[c30don]

hello i am a bit gutted as after replaceing my auto gearbox on my car, its not engageing gear again for more 10 seconds again, which exactly what was wrong with it b4.
When i first fitted the gearbox it drove excellently for a good 20 miles and then i parked it up then i went back to it a few days later and the same problem occurred,,,