8090 Button Mode Error?

Hi,
I’m new to this forum as I’ve recently purchased a VM8090 relay board which seems great so far. I’ve written some vba code to control the board from Access and everything works as expected except when I try to set button mode.
From the protocol manual, I should be sending command 21h with mask=0, param1=0 and param2=255 to set all relays to timer mode but when I do this, only every other seems to get set. If I change param2 to another value (for only one or several of the relays), it works as expected.
Has anyone else encountered this error - it also happens when entering these values in the developer console on the Velleman demo software so it’s not related to my code.
Thanks.