Ruger Mk.3 bolt does not lock open

[Ronz10]

Hoping for some advice. This .22 is 7 years old, and has worked fine until recently.
Using 4 different mags, now most often upon firing the last shot, the slide/bolt does not stay open.
Most annoying, having to check ang use the bolt catch to lock it open to change mags.
Has anyone experienced this and can offer advice?
Thanks

[sawbill]

Not familiar with the Mark 3 but have you considered a good cleaning?

[bdog]

clean your mags also, not just the gun