Why VB refuses to implement my Automation-compatible interface, or call a method in my Automation-compatible interface?

If MIDL hasn't given you a warning that your interface does not conform to the Automation specification, chances are you've used one of the following newly adopted Automation types:

CHAR
WORD
DWORD
INT
UINT

VB doesn't support any of these up to and including VB6 SP4. Your only option is to redesign your interfaces with the following older Automation types:

BYTE
SHORT
LONG

As for INT and UINT, you don't really need them anyway - use LONG.