TP typing a keyboard shortcut doesn't always action


I'm having an intermittent issue whereby TP typing a keyboard shortcut (Alt-C) doesn't have any effect.
This is with TestPartner version 6.3.2 build 352.

I have a Web page (QLV4) which opens another browser window (Print Invoice)
& after TP closes Print Invoice, TP presses Alt-C on the calling page

the TP script lines are:
IEWindow("Microsoft Internet Explorer").Attach
HTMLBrowser("QLV4 HTMLBrowser").Attach
HTMLBrowser("QLV4 HTMLBrowser").Type "{Alt C}"

'QLV4 HTMLBrowser' is in the object map with caption significant, 'QLV4' - it doesn't match 'Print Invoice' so I don't think it's typing in the wrong window

TP result log shows it did actually type Alt-C - the AUT just didn't do anything

to further investigate, with Print Invoice open, I pressed Alt-C on QLV4 - it behaved like cancel normally would - so issue doesn't seem to be caused by Print Invoice closing slowly or anything like that

it works sometimes, but not always

Any ideas, explanations or suggestions please?