The plungers stuck, and that sounds like you've probably just scratched the fook out of the internal bore
This will definitely be the issue, the plate should be horizontal/flat and happily lift up then naturally drop back down into position.

I thought you'd already swapped metering units? Perhaps your original was actually ok - theres equally every chance youve got 2 knackered ones - they dont like sitting!

They can be opened up and sometimes salvaged if its just grime or fuel varnish, but if the bore is scratched its as good as fit for the bin, they are machined with the plunger to have an exact fit, you can't switch them about, it wont seal

If you were closer I would have just come and had a look - i hate diagnosing/explaining K-Jet over the internet, its an arse of a system to describe