I'll post this here because I didn't know why cultures were failing until it happened.
Some of the components used in growing phyto go bad over time.
My f/2 mix from Florida aqua farms grew me many many great liters of T-iso & nanno over 6-8 months. But then in the last month, I couldn't get any culture up and running.
The f/2 mix is susceptible to bacterial degrading over long time. Some products explicitly list expiration after months for this reason.
Another thing that I didn't know is susceptible to bacterial degrading is the sodium thiosulfate that neutralizes the bleach after sterilization.
On another note, every batch I'd ever cultured up from Phycopure Copepod Blend has produced T-iso and grown parvocalanus pods very well.
Except the last batch, which grew.....
...rhodomonas!