Dimmer flicker is a tell tale sign of signal lose/degradation. My boss once used a 25' mic cable into a dimmer pack and then a 100' to another on the other side of the room. The first pack was fine, the second would flicker. This is of course after the arguments over and over again about how DMX and Mic cable is different. He learned the hard way. Now he doesn't question me specing 3 pin and 5 pin DMX as well as many splitters as I see fit/need.
I can tell you in the example above, I put a splitter after the first dimmer pack in line and the other dimmer pack worked fine. It was able to boost the signal enough to get it through 100' mic cable with no problems. However, I must stress that proper cabling is key. I have no doubt in my mind that if I had another fixture/pack chained after the 2nd dimmer pack, it would have had issues as well.
Also keep in mind that the more you add to your chain, the more it saps DMX signal strength. This is most likely the reason for one or the other working, but not both combined. Some toys also don't play nicely with others, and the splitter will all you to break them up, solving problems and quite possibly cabling as well.
As for the signal strength issues with dongles. It is basic electric physics. The chain is in series, however the fixtures are in parallel. Being in parallel, each one receives the same voltage, but different amperage which is where the signal comes from. Voltage is like pressure in a hose and the amperage is the water. No pressure, no movement of the water aka amperage. Now the reason it gets weaker over distance is do to cable and connection resistance. That is in series. Things in series receive the same amperage, but different voltage. So you have your water, but not enough pressure to move the water. Since dongles put out less voltage then consoles, the chain can only be so long and/or have so many fixtures on it.