My atv will not turn over.

I have a brand new 250CC ATV...
Almost since the day I got it I have had trouble starting it...
It sound like the battery does not have enough power to move the starter fast, it moves and turns BUT it is slow and struggles....

I have had the started tested at the parts store and they say it is fine...
The only thing I noticed is then I try to start the atv the voltage on the battery immediately drops to about 7 volts...
I heard this is battery problem BUT I have purchased 2 new batteries and no change...

any thoughts?  

When you say the battery voltage is 7 volts is that measured right on the plus and minus battery terminals?  That is the only way to eliminate poor connections dropping excess voltage while cranking.  If the battery terminals show 7 volts then that means one of three things:

1) The battery is bad.
2) The battery is discharged
3) The starter is drawing way too much current

To drag a good fully charged battery down to 7 volts will take around 80 amps.  7 volts at 80 amps is 560 watts.  Do this for a while and your starter is going to get really hot.  Does it?  Do any wires or connections get hot instead?  Try jumping your quad to your car.  Does it crank faster? Does it start normally?

After a few minutes with the quad running at fast idle, measure the voltage right on the battery plus/minus terminals.  It should be 13.5 to 14.5 volts.  If it reads 12.6 volts or less your charging system is not working.