Unable to dock to ships attached to an asteroid

[Theseus]

For the last couple of hours I've tried to dock a fuel ferry to the asteroid base I am building around Minmus, but the clamp-o-trons won't connect, even though I get a perfect fit on the Dock Align mod. I've tried docking the ferry to two different vessels on the asteroid.

The two mods I have installed are MechJeb2-2.5.6.0 and DockingPortAlignment-6.2. and the version of the game is 1.0.5.1028

I've checked and double checked that all the ports are facing the right way.

I've tried so many times I usually get a reasonably good fit every time.

Tried on another vessel on the same asteroid - same result.

The debug log keeps repeating the same line without any change when the two docking ports connect.

What is going on? Do I suck so badly at docking or is it not my fault?

Thanks for any help.