i have a single wire cobweb antenna with the 1:4 balun had it up rained heavy one day wouldnt tune took down checked all connections all good no water ingress etc tested on ground with vna all in spec put back up worked a charm for several weeks through rain also, now ive lost tuning on 20/17m checking on vna its not bad 2.3 swr so should tune but rx way down like dead short, going to take down at weekend again but stumped as to what is causing this as im expecting the same coax dry/balun dry also??

Element insulators.

The bits that tie the ends together...

Yes, if it is braided especially. (soaks up a lot of water)
Let it dry and spray it with waterproofing spray (there used to be only a couple of makes, now there are a lot) letting it soak in well.

The cobweb does detune in rain though, but usually recovers when it has dried off.

the ends used to tie up are like washing line but without the steel inner support strand the water seems to wick off it ok, tuning hasnt returned and rx way down now on all bands im thinking must be a balun issue

Doesn't sound like a normal rain problem.

But check the plugs at the shack end, and for continuity across the coax from the antenna, and if you have an analyser a sweep from 14-30, get as much info as you can while it is still up, maybe save it going up and down to test later.

How is your balun constructed? Coax or wire?

wire balun, vna doesnt show it overly bad but ill do a fresh sweep now see what it shows

The "original" balun was wound on the core using two pieces of thin coax in parallel.
There was a balun on ebay at one time wound with black and red single core wire.

Too much power into these and the insulation would melt.

Didn't happen with the ones wound with coax, as the insulation (PTFE) did not melt, the SWR would creep up as the core heated though.

If the wires on the wire wound one shorted, you would have dips in roughly the right places, but they would be higher then they should be.

The same can be the case with a poor connection, the trace would look roughly right, but the dips would be much higher.

If there is a drain hole in the balun box, I doubt that water will be the problem.

When you test it, take the coax out of the rig and test it there (My antennas come into a switch first).

I use high strength monofilament fishing line for insulators...might be worth the swap while it's down... I'd also dummy load the cable and see what it does under power, as this sounds a lot like weather ingress to me.

Could well be, it doesn't sound like a wet cobweb.

I'm sure that if all the tests mentioned are done and results recorded the answer will be found in the process, and if not the results will enable someone to give the answer.

PL259/SO239 are really not a good idea for outdoor use, unless very carefully protected.

Thanks for replies hoping to drop antenna on Friday and check balun

Try a new coax feed-line as my suspicion is water is penetrating into the coax.

just quick update finally got break in weather to drop cobweb, as expected coax dry and testing good and balun dry, swapped out balun for one from hamgoodies.co.uk and all good again cant really see anything obvious wrong with old balun but will investigate more however the coax used on the toroids doesnt look great quality, as a side note i know cobwebs arent known to be great on 12/10 which i can really notice is there anything can be done improve it here?