I'd say take the clutch cover off and make sure that the pressure plate is pulling down tight against the pucks when you let all the way out on the clutch handle.
Sounds like you may possibly have something that is stuck.
Did you have the clutch lever locked back so the clutch was disengeged while it was stored up?
If so this could be why it is stuck in a way that wont let the clutch engage with enough pressure not to slip.
After taking the clutch cover off, squeeze the hand lever to see if the pressure plate is moving out away from the pucks and if is and also goes back down against them after you release the lever, if it does I would take a hammer and a small block of wood or another object that is softer than the metal, like screwdriver handle ect. and then put the wood/ plastic handle on the center of the star not that holds the pressure plate on and hit the wood/handle with the hammer just hard enough that it might knock it loose if it is stuck, It's possible that the shaft that that runs through the clutching system is stuck from old hard grease or rusted somewhere.
It is also possible that the bucking bar or 8mm ball that pushes on the shaft is stuck which is accessed by taking off the sprocket cover on the left side of the engine, if none of this is stuck then you could have a broken main clutch spring or it could have lost some of it's tension if the clutch lever was left locked back with full tension on that spring for a long period of time.
Check all the things I mentioned here and see if of this helps, I hope it's a simple fix and I'm suer someone else on here may have other ideas for you to try as well, this is just the first few things that come to mind as to what it could be.
Good luck and let us know what you find out on this.
Peace