Test craft to demonstrate a stock bug.

[Galane]

This craft is all stock parts http://pastebin.com/dA1Prjny

Put it on the pad then poke the spacebar to launch the probes. Note that the engine on the second one does not activate.

Revert to VAB and only shift the positions of any two of the stages in the stack. Don't alter the craft itself. Launch again and you'll find that all four probes launch properly.

There's a bug somewhere with that quad adapter. I found this back in .12 (the first KSP version I used) and it's still there in .90. I tested this example craft in .25 and .90 last night to make sure the bug shows in both.

It should not matter what order the couplers and engines are placed on the adapter or what order the stages are arranged. As long as the correct icons for the engines and decouplers are together in the stages, the activation should always work.

Complicating debugging this is that Sepratrons are not affected. I tried swapping on small I-beams with a pair of Sepratrons in place of the liquid fuel engines, tanks and probe cores. The Sepratrons light every time, in all possible arrangements.

I've experienced other oddities when using this adapter in rockets, which can be worked around by altering the order in which parts are attached and/or staging order while achieving the same final construction. If the bug is fixed, there would be no need to do such fiddling because there wouldn't be the possibility of stumbling upon a configuration that does not work but should with this adapter in the build.

The Tri and Bi couplers should also be examined to see if they have the same or similar issue.