I have varnished teak, which people always say is the most difficult.
I put the first coat on 4 years ago, with a minimum of 7 that first year, then 2 coats a year since, the lustre keeps improoving all the time. I always thin by about 10% which makes it easier to apply.
I suspect that you are not putting enough coats on initially. anthying less than 4 is just a waste of time.
I use either blakes or International, which ever is available.
Try Coelans, german product, expensive, but brilliant stuff, I use it on teak and iroko on my boat in Portugal and the med, two years and not a mark on it. The blurb says it will last eight years in med weather from what I've seen already I can believe it.
Saw this stuff at the SBS - looks very good and can even be used for coating rubber/plastic etc as flexible.
They do have a website at http://www.coelan.de/ but it wont tell you anything!!!!
Product is sold in UK by KJ Howells in Poole www.k.j.howells.dial.pipex.com