Colin, if you haven't done so already, buy Steve Maskery's DVDs!!
Are you sure the teeth are pointing the right way and that the blade isn't 'inside out'? You could also try cranking the tension up on the drive belt. Are you tensioning the blade by the saw's scale (!) or by your own method?
How wide are the blades you've ordered? You should be able to run a 1/2" blade with no problems on a 12in saw. Anything wider though, and you'll probably struggle (unless it's got a thin-kerf - which is where Tuff Saws usually come in! :wink
. If you are having trouble tensioning the blade then you could always try ordering the blades 5mm shorter than the specified blade length on the machine (eg. 2235mm where they quote 2240mm). If you're sure you could get a shorter blade on the wheels, in the first place! :wink: That would mean the blades require "less tension" (you wouldn't have to wind the mechanism up as far to get it singing).