Canon flash problem...

.... I brought a used Vivitar 728AF flash for occasional use on a Canon
300D digital SLR...it worked as well as could be expected for a fully
manual flash and produced good results in Manual mode.

However if I put this same flash unit onto either a Canon D30 or D60
Digital camera - although it still fires - is a very low flash and
completely useless. (nowhere near as powerful as the onboard Camera
flash - or the push button test flash).

I don't use external flash that much and don't really want to buy
another...

Can anyone explain why a flash that works as it should on one Canon
D-sLR does not work properly on the other two....