is there a way to tighten pins in tube sockets?

"Thee shower?" Actually; the King James method is during a baptism(full immersion & considerably more involved).

Two of the output tube sockets of my VAC amp don't grip the tube pins tight.

The problem I experience from this is a right channel hum and unstable tube bias.

Do the power tubes plug in and pull out of the tube sockets fairly easy?
If so that would indicate poor contact pressure.

Yes you can tighten the tube socket contacts as others have described above. Problem is will the contact pressure against the tube pins stay tight.

Heat on the socket contacts produced by the power tubes + even more heat produced by poor contact pressure between the tube pins and tube socket contacts more than likely has caused the tube socket contacts to loose their tension memory. If that is the case even if you tighten the contacts they will not hold their form, pressure, against the pins of the tube.

Best to have the sockets replaced.
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The hum and unstable bias just might be due to poor contacts on the pins. Bend them a little if you can, then get some contact cleaner and clean the pins and sockets. Put the tubes back in and you may not need to send these to VAC???