Welcome! 
You almost certainly need some zigbee repeaters. While most zigbee smart bulbs Will act as repeaters, passing along messages for both each other and other zigbee devices on your account, Sengled is unusual in that the manufacturer has decided that none of its devices will act as repeaters. So you have to have some other brands’ Devices in order to “strengthen the mesh“, a fancy way of saying that there will be enough devices who can pass along messages that they don’t get dropped.
Start by reading post 11 in the following community FAQ, then go up to the top of that thread and read the whole thing and you should have a much better understanding.
I will link directly to post 11. The topic title is a clickable link.
Also, how many total Sengled devices do you have? You can get into a sort of “musical chairs“ situation with zigbee Devices because they do a check in about every hour and if they don’t find an available “parent“ they will drop off the network. Each repeating device can parent anywhere from 3 to 12 children, typically 3 to 5. (The hub can handle up to 64, after that it depends on other repeating devices.)
Anyway, if there are too many children and not enough parents what will happen is that when one drops off the network, another one may grab an available parent, and this can happen kind of randomly so that you keep getting different ones going off-line.
The solution to this is the same as the solution to the range issue: adding more zigbee repeating devices to strengthen the mesh.
BTW, the Ikea Tradfri plug in pocketsockets are inexpensive, available in many countries, and happen to be very good zigbee repeaters in a smartthings set up. You don’t need to get the device that says it is a repeater: just the regular smart plug will do. So those are often a good choice if you just need to strengthen the mesh.