lincsman expressed precisely :
I have recently fitted an adjustable Duo Flush Dudley Turbo 88 in my
toilet cistern. I can not get it to perform a half flush on a regular
basis. Has anyone out there fitted one of these and had the same
problem? It is driving me mad as I have tried to adjust this so many
times. All help would be appreciated. Many thanks.
lincsman

Lincsman, if you are still around....

It is rather counter intuitive, but try reducing / lowering the cistern
fill level a bit.