Sengled bulbs are very unusual in that they do Not act as repeaters at all. This is intentional on the part of the manufacturer, who was concerned that people turning off a bulb at the switch might then lose a repeater from the network. So in terms of network planning, they are exactly the same as a battery-operated sensor. They will take up a child slot on a parent device, but they will not repeat.
In your case, it sounds like you may have run into a problem that can sometimes occur in zigbee, which is that of orphan devices that cannot find a parent because the network configuration has changed since they were first added and there are now no child slots available on a repeater near them. ( this is also sometimes called “bottlenecking”.)
You may have the same devices in the same physical locations, but because they have now been added in a different order to the network they have different parents and sometimes it’s sort of a musical chairs issue where now somebody gets left out.
This can also happen if a particular repeater goes bad and then its former children can’t find a new parent. 
If you’ve been adding a number of new devices, you may also run into the hub’s own device limit for directly connected devices.
Unfortunately, since SmartThings does not give us any routing utilities for zigbee, this can all be very difficult to troubleshoot on your own.
My suggestion would be that you just get in touch with support and let them look at it and see if they can see anything from their side.
Otherwise you’re going to have to go through a very tedious trial and error process of removing devices and trying them in different locations and seeing if you can find a layout that works.