birdshot said: The Virginian, is correct. The first motion of a fired pump gun, recoil sends the gun back, the forearm is in your hand or resting on something, this causing the forearm to be pushed forward, which unlocks the bolt. Now with about 7 lbs of gun moving toward your shoulder with say 30 foot pounds of energy, you stop that motion, except for the bolt which was unlocked when the forearm was pushed forward or more accurately left behind, the bolt continues aft ejecting the shell.
I just took my 22 yo. 500 and after simulated firing on a snap cap and dropped it from a measured distance of 24". 5 cycles of fire it and drop it from 24" vertically onto its butt on a hard floor and the bolt would not open all the way let alone eject the round. To get consistent ejection, the gun had to be dropped from 30".
This is a situation based on sudden stop and not a persons body recoiling with the system. An object falling from 24" is moving at 11.35 ft./s. A 1 oz. load fired at 1300 fps in a 7.5 lb. gun gives a recoil velocity of 11 ft. /s. An object falling from 2 1/2 feet is moving at 12.7 ft./s. To achieve 12.7 ft. /s you need a 1 1/8 oz load at 1300 fps in a 7.5 lb gun. Again 12.7 fps was the minimum to get the snap cap to consistently come out of the gun and land next to it. The OP didn't detail the shells he was using however we have baseline figures to work with here.
If a broken in Mossberg barely performs as described by the OP in a sudden vertical stop from 2 1/2 feet how is it supposed to do it during a more attenuated stop horizontally? In a horizontal stop gravity is not continuously acting on the bolt / slide assembly in a manner that will be conducive to opening it; vertically gravity is still trying to accelerate the freed assembly down. On the bench the persons body absorbs some of the free recoil energy and the rate of deceleration is seriously attenuated; a drop onto a hard surface gives a much more rapid deceleration.
Still sounds like a good time to have it looked over. If nothing else, it is a free check up anyhow.