I have only been on varac a short while but scratching my head over something.
A ham friend of mine about 50 miles from me just loaded the software also. He can call CQ get replies he can have people call him and he can chat and he can call other people and connect.
The same holds true with my station. I can call CQ and get replies, I can connect with any other station, but for some reason I cannot connect to him and he is unable to connect to me. This is with S7 signals between us and tried on different bands.
We both run SDR, here is on a Anan 7000 and i am on a SunSdr2.
The TX/RX delay settings wont do too much. The trick is to reduce your DSP Filter size in Thetis which has the biggest impact on latency. There is a similar setting in the Sun and Flex. With these set to minimum QSO's between SDR's work fine, I have had many between all 3.
We actually juggled tx delays and. Rx delays tonight to no avail. I think we tried around 15 to 20 mS and have gone down as much as 2mS with no difference. We're can hear one another fine i might mention.
Maybe NC3Z has the trick.
Sam / NR4A
Sounds like an SDR delay issues that when combined (both sides rune SDR) they accumulate in a way that it goes beyond the VARA threshold. There are ways to minimze those but ill leave this to @Nc3z who specialize in VarAC &SDRs.