I can't see other Ultralight traffic,even when they have S-Mode transponder

I am using Safesky and I can see ohter GA planes. Strangely, most Ultralight do not appear. I have tested flying next to an airplane with a Funke TRT800 S-Mode transponder in mode Sierra, and it was not visible in the app.
Could this be an issue with the Transponder type?

I have the same trasponder.
It as S-ES capability, but you need to feed it with a GPS source in order to trasmit a ADS-B positional message (about 1 per second)
Without a GPS source, it only replay to mode A,C and S interrogation from SSR. Using some groung stations, the aircraft position is determinated using MLAT (multilateration) tecnique, but it needs at least 3 or 4 stations receiving mode S replay from same aircraft to calculate its position.
So, when an aircraft wit a TRT800-S trasponder not feeded with a GPS source, is at ground or fly at low altitude in an area without enough ground stations, you cannot see it on Safesky, I think.