New flush valve handle/ too tight / too loose/ not springing back

[SJRSJR]

Hello

I’ve just had to fit a replacement flush valve on a toilet with a lever handle. Previously it had a mechanical arm but this is a dual flush valve with a cable.

When I hand tighten the inside nut towards the handle, it’s too tight to flush, when I loosen it, it wobbles. Either way, after flushing it doesn’t spring back unless I put it back.

Normally I’d expect to be able to figure it out but I’m stumped by it. Any tips on how I can set it so it flushes and springs back.

It’s a Flomasta Dual Flush Valve. Thanks

 

[Wayners]

I wonder if it's not sitting flush. On a curved surface?
I don't like that sort tbh.
Normally works fine though