Hello, non tech guy here… Why do my Sengled LED Zigbee lights work when my cheap Peanut Zigbee smart plug is plugged, but when I switch the Peanut to a Sonoff Zigbee plug only one of three work? (all three are in the same ceiling fan, so same distance from plug) Thanks
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First rule of home automation: “The model number matters.”
That said, one possibility has to do with how Zigbee devices relay messages for each other.
The Zigbee profiles that smartthings supports all handle this process the same way: at the time that a new nonrepeating device joins the Network, it is assigned a “parent” who will handle passing its messages along to the hub.
Although most Zigbee smart bulbs are repeaters themselves, that decision is left up to the manufacturer, and Sengled has decided that none of its smart bulbs will be repeaters. So each bulb has to find a parent to help pass its messages along.
Another aspect which is left up to the manufacturer is how many children each potential parent can have. Being a parent does require some resources like internal memory. Anyway, it’s typical to find that Zigbee home automation devices of the type that work with smartthings can parent anywhere from 1 to 5 children. And again that decision is made model by model by the manufacturer.
From what you describe, it sounds like the new smart plug handles fewer children than the old one did. That leaves the other nearby devices in search of a parent as “orphans“ and they may not be able to get their messages out to the hub. The next time the orphan tries to rejoin the Network it will request a new parent, but if there isn’t one within range, it’s out of luck. 
The easiest way to test If this is what is going on is just to add an additional repeating smart plug of the same brand that you were having trouble with in the same room and see if that solves the problem. So that now there are two potential parents in that vicinity, which means more children can get “adopted.“ (this process may not happen instantly, unless you first remove the lightbulbs from your network, then add the smart plugs, then add the lightbulbs. There’s just some variation in how long it takes the orphans to try to rejoin.)
You can also contact technical support for each of the smart plugs and ask them to check with engineering and find out how many Zigbee children that particular model can have. Don’t expect frontline support to have any idea what you’re talking about, but there will be somebody there who does. 
Anyway, speaking from a field tech perspective, that would be my first guess. You orphaned the bulbs by switching to a different parent that didn’t have as many child slots available. But that’s just a working hypothesis, I don’t know for sure. 
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Thank you for that thorough response. I did plug in another Sonoff Zigbee smart plug and had the same problem. Only one of three bulbs connected. I went back and forth several times between my old Zigbee plug and the new. All bulbs connect with the old Peanut Zigbee smart plug, but only one connects with the Sonoff smart plug (same if I plug in two Sonoffs in the same room… Only one bulb will work). However, in a different room I removed all plugs and only one bulb works (which means it’s connecting to a repeater outside the room or the main hub, but when I plug in a new Sonoff Zigbee smart plug in the room all three bulbs work. So, now I’m really confused. The Sonoff will work three bulbs in one room but only one in another. Thanks
How much time did you give the bulbs to find the second Sonoff in the room? Speaking just as a field tech, I probably wouldn’t check that until the next day. The stuff isn’t instantaneous. But it’s likely a minimum of two hours at least.
Or did you actually remove the bulbs from your network and then re-add them after the second plug was in place?
Also, did you reset the second sonoff plug and then install it in that room? If you just physically moved the location but didn’t go through a reset, then it might still have children from its old location even if it couldn’t reach them.
Anyway… Given how few tools smartthings gives us to diagnose network problems, it may just be enough to know that the one model works and the other model doesn’t. If it’s a limit on the number of children a device can have, you can’t change that anyway.