did they work well before? did you notice the powerline light status when they disappeared from the tpPLC utility? they are on or off or something else? 

Meanwhile, to avoid the interference of the powerline environment, you can plug them into the same room or exchange the location between the powerline adapters and the remaining 2 units. 

Furthermore, try  to disable the power saving mode of the powerline adapters and give it a go. https://www.tp-link.com/en/faq-2018.html