Holy shinto folks...... no wonder this forum erupts into fights so often. Half the time I think its cause no one reads what people are saying.....anyhow.....
OP: You said "concrete blocks". Would be good to know what you're referring to and more importantly, how tall of a block/riser you need to get the clearance for loading?
Like one of the few that understood what you said suggested, wood boxes. Just be mindful of how much they stick in-board of your front jack feet as you don't want your tires hitting them when pulling out/in. If it's not a lot you need, perhaps a stack of 2x6's or 2x8's, screwed together, would be enough of a foot print to be stable enough.
I have unloaded my camper at times on concrete and have had the sudden "kick out" you sound like you're describing. Not quite an underwear change moment, but does make you blush....
๐ If you're worried about them kicking off the stacked 2x's, blocking in the feet, as someone mentioned, would work. 3/4" plywood strips would do the trick. Or some alum. angle iron if you want a bit more Gucci factor. Or if you want quick and dirty, 3 or 4 #14 screws to trap the foot would work. Just lower the legs down far enough so you can shove the risers under and get them lined up.
If your ground is soft, run a 2x from the fronts to the backs and between the backs. That'll increase the weight distribution a bit.
And anything wood on the ground needs a few coats of good paint.
:h 1998 GMC 2500, 10.5 Okanagan, My better/smarter half, George and Finnegan(APBT), all I need.