My bow is sliding about on the strings.. as if it's not 'biting' to make the string vibrate. If the string does vibrate it makes a harsh sound first and I have to press really hard. Sometimes the E string doesn't 'sound' at all but just squeals so I changed it and it's a bit better I think, although it has still squealed a little

I've tried changing rosin to a softer one but that hasn't helped.

It can't be my technique can it, because it didn't happen earlier this year

My bow was new in March. Could it need re-haired already? My strings are also under a year old and I haven't been playing heavily this year.

Okay I'm really annoyed and confused

(p.s. yes, I have insomnia tonight grrrr)

Have you try asking your teacher try bowing on your violin to see if the problem reproduce?

Try this exercise: Bowing on open strings, begin each up/down bow with a a stronger "string gripping" stroke and then relax and bow, see if the problem still exist?

I would second the suggestion to ask your teacher to play it before changing anything.

However...

I bought my daughter a new 3/4 size cello last October. When we came back from our summer holiday her bow was sliding all over the place, which I put down to her not having played at all for seven weeks. At the end of her first lesson in September her teacher said that her bow needed re-hairing. I found this most surprising as the bow was not even a year old, my daughter only practises 30 minutes a day and she had had her previous 1/2 size cello for three years without the bow being re-haired. Somewhat reluctantly I followed her teacher's advice and had the bow re-haired and the difference was amazing.

An exercise about bow pressure that I like to use -

If you play the harmonics half way up the string and use as much pressure near the bridge as they will take but stilll sound. Then try using the same amount of pressure on an open string.

Hope this helps.

I'm probably way off the mark, but is there any excess rosin on the strings themselves? (I doubt it, having an idea of how attentive you are to Matha, Lottie! )

Bow grip is (excluding individual technique) a combination of rosin and bow hair. Apparently bow hair get "worn down" eventually, so the hair becomes completely untextured and flat wich means there is nothing left to grip onto the string. Obviously rosin wont help here - it wont stick to the bow hairs.
I think that's right. In other words, if it's not lack of rosin, it's re-hair time. Or open strings practice...

Also just because you bought your bow in March, you don't know how old the air was on it (unless it has just been made in a factory I guess) - I needed a rehair straight away when I bought my 2 most recent nice bows as one had been tried in a shop by many people and the previous owners of both clearly didn't get them rehaired often. I rehair my regular bow every 6 months or so, although I do play a lot on it (40 hours a week or there abouts).

Thanks for all the suggestions.

I don't have a lesson until next week so I'll ask my teacher to try it then. But it does sound like it might need a re-hair which is annoying as it means sending it away

Perhaps my teacher will let my try one of her bows to compare.

But I will try the bow and pressure excercises - thanks

I know you live in the "middle of nowhere" but re-hairing does not take very long. I dropped my daughter's bow off and picked it up a couple of hours later (I had pre-arranged this with the luthier) so if it does need re-hairing perhaps you could arrange to have it done the next time you go "somewhere big."