I was having trouble thawing a few layers in one viewport only. I found a workaround, but I'm posting the problem in case someone has more insight, or if the workaround helps someone else.
I drafted several lines and features in a series of layers in model space. The layers were all named UTIL_XXXX. For example UTIL_SURF, UTIL_DRAIN, and others.
I had previously created a layout tab. There was a viewport in this layout tab. Although the UTIL layers were thawed in model space, and the "Current Viewport" for those layers were thawed (using EXPLAYERS, aka the "Intellicad Expolorer Layers" button) in the layout tab, the layers did not appear in the viewport.
I tried closing and reopening the drawing. I was able to "draw" lines on the UTIL layers in the viewport, but they were invisible.
I found that using VPLAYER, I was able to list the frozen layers in that viewport. They did include the UTIL layers, even though the Layers dialog showed them as thawed. The VPLAYER command allows you to thaw/freeze layers also (by typing the names in by hand) and this did actually thaw those layers.
There was, by the way, a second layout tab in the same drawing, with another viewport that did not have this problem. The UTIL layers were visible there.