Having trouble consistently keying radio remotely on latest SmartSDR for Windows

Firstly, thanks for reading this, and for allowing me access to this resource pool and to your support network.

I've searched the forum quickly for similar problems, but without much luck.

I've pursuaded my local club to purchase a used 6400M. It's been supplied by another member who has a lot of knowledge with Flex and with both MacOS and iOS Flex client software, but not with Windows.

The radio is set up in our clubhouse, and we will mainly access it remotely.

Three club members are currently testing the radio. The radio supplier, myself, and another member.

The supplier is having no problems, using Apple products. Myself and the other member are having an issue. We are both using Windows 10 (fully updated), and are both having the same problem independantly. Both of us are relatively tech-savvy, but after two weeks of head-scratching we have hit a brick wall and need some help.

Regardless of any settings we seem to choose, audio profile, TX profile or anything else, intermittantly, when we hit the MOX button, we see our audio represented correctly on the input audio level meter in SmartSDR, but no SSB carrier is transmitted. When we try it five minutes later, everything is fine, and so it goes on. It's almost like TX has been temporarily inhibited.

We have an 80/40/20/15/10m EFHW antenna on Ant1 and a 100W dummy load on Ant2 available.

If, when it is working, I turn the RF power down to 0, I see exactly the same response.

I also have access to another club's Flex 6600, and I experience the same problem there!

I'm so confused now, I really need some advice.

Thanks for reading, and sorry for the insanely long first post.

Berni M0XYF