If there is not a sticker on the blower, you may have to try and locate the specifications for it. This presumes you know the brand and the company has a web site with product specifications on it.
Knowing the specifications will allow you to judge the maximum amprege draw.
If you can't find the specifications... and if you have another, more modern power supply (350 W or larger), I would try that first. Last resort is to try a car battery... no lack of amps at 12VDC on that baby.
If you still see the problem, and you are sure you are applying plenty of amps, then you may have a bad blower.
As for needing a shroud, are we talking a squirrel cage fan or traditional fan? A picture would help describe the "nakedness" of the fan for us.