I remember listening to one of the early Talking Beasts podcasts discussing VDT, and either @glumpuddle or @rilian suggested that the Duffers were wearing gloves when they were carrying the dishes.  I think this explanation fits quite well, meaning that the gloves would be invisible since they were directly touching the Duffers but the dishes would be visible since there was no direct contact.  Likewise, the weapons were invisible since the Duffers were touching them directly without wearing gloves, fitting with the fact that the arrow was vivike when it left their hands.

My explanation is weapons and dishes are two different things. (Not a great explanation but my own.) Their clothes are invisible too. Maybe weapons count as part of their outfits.

All very good reasons.

I'm not so sure C.S. Lewis didn't do this accidentally - but the explanations are so much better

That's part of the fun, to try to work out what may be the reasons that would work inside the story, in that universe, whatever the technical reasons may be for what the author did in his writing process.