I bet you $11ty-bazillion it WASN'T an
ETC Sensor 98 dimmer rack, nor did the LD have much authority in its selection. ETC dimmers are what good lighting shops have. Period. "Industry-standard" and all that other marketing hokum.
ETC Touring Racks since (1996?) HAVE had a mechanical locking feature as you describe, but it's wise practice nonetheless to make ensure ANY/Every piece of portable pull-out gear is correctly seated before applying power. Vibration from bouncing around in the back of a truck takes its toll on all gear.
Next time, rather than "spending countless hours" troubleshooting, due do diligence. Then, if the problem still exists,
call ETC at 1-800-688-4116,
24/7/365. Tech support doesn't mind stupid questions, it actually gives them a break from the more difficult problems, and both parties are left with a feeling of satisfaction. Unlike with some/most other manufacturers, you won't be told to first call the place where you rented or purchased the ill-behaved device, unless it needs something more than phone service.
Thanks for the tip,
nmccoart5. It's a good one, solving a problem that has bitten me, more than once. Remembering to push in all the modules is a small price to pay for the privilege of using the best dimmer on the market.