If you click directly on the symbol they will sometimes vanish. The trick is to condition yourself to click anywhere else on the line with the symbol - that way they never vanish. For example I always click on the 'last' price since it the my first field to the right of the symbol.
yes, I figured that for the ones that were had some thing to do with the symbol, that continually disappeared vvix, xiv, uvxy, tvix, fidelity stop trading in one of there TD may have as well..
Some ICE symbols are being dropped. They may still appear when you search for them, but they no longer function. Ice started charging a lot for these data feeds and TD Ameritrade has dropped them. The big one dropped is /D . Dollar Futures. They suggest substituting $DXY.
I believe they had glitch in software that sync watchlists between TDA and TOS as a result some symbol disappear because of it. It's also affect watchlist sorting.