Emmons GS10 6th string going flat

[Zach Kerbel]

Hi
Like it says in the subject line. Standard E9 tuning. Tunes up properly but after a bit of playing it goes flat. But when B pedal is down it is still in tune.
Wondering if anyone has any thoughts on this. Was thinking it might be the tuners but I don’t think so. I’m told it may be the return spring needs tightening/shortening. All hex nuts are tight.
Any insights/thoughts appreciated.
Many thanks.

Edit: any pointers on how to fix it also greatly appreciated.

Edit 2: it has new strings.

Thanks.
Zach