Yeah - had a runt that just stopped growing for awhile. It got so far behind at one point I was about to cull it, but decided to try and take it out of the hydroton and check out the roots. Found out the tip of the taproot had grown into this little tiny hole on the outside of a piece of hydroton and never quite made it out of the netcup. It was crazy looking - now I wish I had photographed it.
The runt recovered and caught up after extracting the root. My story has a happy ending too.