Full Lockers???

Are you saying both front axle tandem and rear axle tandem are both locker differentials? Plus an axle interlock?
Just trying to clarify.

You only refer to one switch. Are you sure you don't have just an axle interlock then.

 

also, how did you test before you buy?

 

Not sure of your present "test method" where you now hear them click in, but say that not all wheels are showing to be functionally "drive" (powered) wheels.

Several "angles" to consider having your setup.
Without your inter-axle locked in, only the front axle will be powered. (just to get that part out of the way)
Each axle, when locker is activated, will lock the right side wheels to that same axle's left side wheels. So if your truck axles were jacked up with right side only off the ground, if you try to turn the right side tire it will not turn, as it will be locked to the left side tire on the ground. That will be a basic test for each locker.

Now on to the interlock...
Activate both lockers and interlock. If the rear axle only is jacked up off the ground, it should be impossible to turn the rear tires, if the front tires are on the ground.

Without knowing any more of what's failing in your current "test", these are the basics.
One thing to always remember... NEVER engage or disengage interlock or axle lockers, if any tires are spinning. Rotating with traction at same speed is ok, just not spinning (no traction).