Once flourishing, now vanished

I've been maintaining my tanks pretty consistently for the past couple of years, but over time I have had some plants that thrived for a while, only to later vanish.
I know there are a few folks around here that I provided with ET. Used to carpet my 55, but through no intent of mine I no longer have a shred in my tanks.
Stretching further back, I used to have massive clumps of java fern, so much that I remember sending huge pieces to folks. But over time they slowly died and are now only a memory.
Similarly, I used to regularly wrassle a mass of baby's tears, and now there are only a couple of strands hanging on.

On the other hand, I used to have a large stand of crypt wendtii which died back - but in recent weeks it has been making a reappearance.

Anyone else have similar experiences, where one of your plants will do super for a while and then, without you realizing you are changing anything, it disappears?

Any explanations for why this happens?