MDI G-Code not actioned

Hi - Very new to CadCam - been looking at Mach3 driving a Denford MicroRouter Compact for the past week and today cut my first slot(s) in 18mm Plywood to make a sacrificial table.

I haven't yet run a 'file' but I have been using the MDI and entering G-Code by hand which generally does exactly what I would expect but on occasion hitting [Enter] after typing the G-Code does nothing - not even displaying the command in the pop-up box - and at other times it does clear the line and show it in the pop-up but the command is not transferred to the Denford.

Even when that happens and I test that I can turn the spindle on it does action that and it also responds to [REF ALL HOME].

Once I [Esc] from the MDI Input I can use my hand controller to move the tool along X, Y or Z without problem but the manual G-Code entry still doesn't operate.

I've also made sure that there are no [Error] codes.

??? There must be something simple that I'm missing but look to the wisdom of the forum for guidance :)

TIA.