Ok, I'm not sure how to explain it, but interlocking using the starting objects on Sandbox doesn't work when doing it the conventional way. I've heard that you have to do it differently so that they spawn correctly, but I can't find a guide for how to do it. Not even in Forging 101. Please help.
It's actually relatively simple. Always avoid setting an original object to spawn at start: no, because it will return to the place where you last saved it and become unresponsive to monitor mode when you start a new round. So, the only way to interlock/geomerge is to set the other object to spawn at start: no and then place that the original object in second. This is why I recommend replacing objects with original objects only when you hit the item limit - you won't have to worry about not being able to make additional interlocks into the original object.
Please read the link in my signature for an indepth breakdown of how to interlock using map original objects.