Morning all. Went to get my boys oset 20.0 lite out of the shed yesterday, fully charged, swithched on on the handle bar, 3 lights showing on the battery gauge, and nothing. When I say nothing, you hear the relay make the click noise but when you touch the throttle there is a slight response where the wheel turns a fraction then it cuts out. Repeat process and it does it again but nothing more. 

It was fine when we put it away after a dry ride. Any thoughts???

Try wd 40 in the throttle body, problems i've had include damp in the main relay causing it to seize, but that's obvious because you hear it click if it's ok. Also had the same issue re wheel turning a few times then cutting out. Seems to have been damp inside as wd 40 cured it by squirting in gap at side of body, i always coverl it with a bag if washing bike and made a cover for it from old inner tube to keep most of the rain out. 

They don't like water!

water should not really effect the operation of a motor as water is conductive.. my best guess is that you may have a sticking brush in the motor or a bad connection to the motor ( from controller ) or a faulty throttle ... do it still cut out if you only twist the throttle less than half way ?.  test the motor by directly connecting a 12v battery to the motor leads ( keep rear wheel off the ground ).. 

My 16.0 did this and it was dirty connections in the main fuse holder. Check all connections!

Water affects oset relay and throttle big time, they aren't sealed. Not a problem motor wise but the relay is open to elements and if it gets into throttle body can cause problems although wd40 can help( been there had both issues)

Yes water can effect all the lower power side of things but the fault that is described with the startng to spin up then stop it is very unlikly to be caused by water, more like a diry/bad connection somewhere. And has already been said the fuse holder is a very good place to start. Sticking or worn brushes can cause these symptoms but  that would not be the first place to look .