First thought would be what speed are you set to?
Second would be how well balanced is the work piece, even 450rpm is quite fast if the load is unbalanced. bigger lathe bigger bits of timber but if I am turning a 10" say 1" off centre I have the luxury of starting a 0rpm and turning it up gradually have turned off balance pieces at around 250rpm despite my lathe weighing about 10x yours.
I suggest you get a balanced piece 4" to 6" in the lathe at 450rpm with the lathe turned off spin the wood by hand and mark the top a few times, if it is balanced then the marks should be random as there will be no heavy part pulling to the bottom.
Give that a try see if the results are any better.
ALL lathe manufacturers overstate what a lathe is capable of, the DML250 says it can do a 10" bowl, but that will be in ideal timber perfectly mounted, mine says it can do 68" but I would probably want to be outside the workshop when it started spinning that!