I've had my 750k3 frame up on one in storage for the last decade Recently when I brought it home and closed out the storage unit I noticed the jack didn't want to lower. I wound up spraying the pivots and jack cylinder ram with WD40 and pumped it up to its highest position, another spritz on the ram and I tried standing on it and pressing the release pedal with a broom handle, it worked slowly but it worked. Make sure the safety stops are clipped in their out of the way position.
You can always use the ratcheting straps that came with it to apply some pressure so you don't have to waste your day standing on it waiting and hoping.