Aircraft not being shown

I am trying to track a friends airplane. He is on a squawk code but FA does not show it. Is there a reason?

I have been having the same issue on recent flights (4 missing in the past week). Is this a known problem?

They must have a Mode S or Mode S/ES(ADS-B) transponder to track on Flight Aware.
UAT 978 OUT would work if someone in the area is running DUMP978.
Mode A/C transponders are not currently tracked as far as I am aware.

(Edit this was wrong).
piaware can track mode A/C.
There could have been an issue with not enough MLAT devices or a problem with the back end servers.

My plane and my friends plane were always displayed until recently. Both are Mode C only.

FA may be using another system for Mode C aircraft. Piaware receive only on 1090Mhz.
Dump978 can be added to receive 978Mhz UAT.
Mode A/C use 1030Mhz.

I would expect that a flight plan would be required.

The plane is on a flight plan

I am mode C only. Until the past week, all flights on a plan or with flight following have been tracked. If mode s is the new requirement for FA, it is brand new.

It sounds like another system is down or not working well. Piaware only uses mode S S/ES