Little Boy was Uranium and a gun type device.
Fat Man was Plutonium and an implosion device.
tl:dr
Gun type is far simpler, was never tested, it didn't need to be. But it only works with uranium not plutonium.
For plutonium you need an implosion, which is technically far more difficult, thus they tested it at Trinity.
Further, the difficulty with uranium is separating enough 235 from the 238 they had mile long buildings to do this and still only had enough for one bomb, so no testing. Once you have a reactor (fermi) plutonium isn't that hard to get, but creating a nuclear detonation is far trickier than U235.
ETA: Fuck that now looks like I'm going for the Sir Creep spamming award.