I can't work out why I can't advance rank to swordsman in the fighters' guild. My strength is 50 , my endurance is 45 and I have short blade skill of 63 (all other weapon skills are 5 or over).
Can anyone advise me as to where I am going wrong?
Thanks!

Short Blade is Not a "favored skill" of the Fighters Guild.

As jeabo has mentioned you're missing the primary skill. For Swordsman you need a 30 or higher in one of the following;

Axe
Blunt
Block
Heavy armor
Armorer
Long blade

Oh right! Thank you both. Dratted Fighters Guild not recognising short blade as a skill!!

Short blade is favoured by those who use stealth. The Thieves Guild and Morag Tong that is and the vampire clan of Berne.
The Fighters Guild is more straight forward. Same goes for the Imperial Legion.

*sigh* Yeah, I feel your pain, OP. The big manly men n' gals over at the Guild think that they also need manly equipment to be just that dang manly fighters. Unfortunately, they still get their manly asses whipped as the Telvanni wizards zap them a bit from the distance.

It's a "macho" thing I guess....