The VAST majority of gas struts have more pressure in them than can be compressed by hand. (Anything more than 150N) The outward force is what provides the "lift", if you could push them in, they aren't going to lift much weight.
If your struts were once collapsing under load, but suddenly find they are rigid and won't move, the most likely scenario is that the FITTINGS they are attached to have come loose, and the struts are not lying in a straight up and down position. In this case, they will not collapse but work more like a "prop". Tighten all the fittings, and test them again.