I believe the guides really only drag as far as the edge of the slide. You
might try moving your mouse around your slide edges and see if you can pick
up the guides there. You might also try setting your zoom to 25% or so -- it
may be easier for you to see and grab the guides in the grayspace off the
edges of the slide.
Seems that dragging the guides off slide is the way to delete them, all but the
first one each of vert/horiz guides. Example:
Start with one vertical guide.
Ctrl+Drag to dupe it; repeat until you have the max of 8 guides.
Now drag them all off slide.
Drag one back on. No problem.
Drag another one back on. Nuh-uh. Not happenin'.
But you can now Ctrl+Drag do dupe the one guide you dragged back from the dead.
Since we were already at the maximum allowed number of guides, that tells me
that we've deleted some.