Hi I have had some problems with my new lens and wondered if anyone had similar problems and any solutions?

I recently bought the Tokina 11-16 pro II after much discussion on here. A few test shots at home and it performed pretty well.
My 1st outing was for some UWA shots at my local beach at sunset. The weather is pretty unpredictable where I live and one minute it was calm, the next minute gail force winds so i packed up and went home. However since then i have had problems.

Initially I couldn't get the camera to fire (You may have read an earlier post on here) but having taken it into a camera shop and showed it to them I realised it works in manual focus mode, just not autofocus. I hadn't worked out you need to push it back to get into manual mode! Having barely used it before I am not sure if the autofocus ever worked or if I got sand into it on that windy day. The manual focus adjuster did feel a bit gritty until I used my sand/dust blower. However the autofocus problem persists.

I got the pro II version specifically because its newer design is compatible with autofocus for the Nikon D5100. Yes I can still use it in manual mode but I find this difficult for landscape photos - much easier for close ups when you can see easily you are in focus! I have taken several blurry photos now and its pretty frustrating!

If it is the sand, can it be fixed? Do I have to send it back to the company I bought it from or are there any independent companies that fix cameras? I would have sent it back already but they quoted at least 4 weeks and I am off to New Zealand in 4 weeks - the main reason for buying the lens! Also if it is sand then Im worried they will refuse to fix it and I will have sent the camera away for >1 month and paid for international shipping for no reason! Eglobal have not been particularly helpful so far!

Also is there anything I can do to help it? Or any tips for manually focussing for landscapes?

Thanks!