I am also having this problem. I can only edit plants I made with the older version of Vectorworks, the "Include on plant list" box is checked and I still cannot make or edit tags on the plants in question.
It's making the program useless to me, since 100% of what we use Vectorworks for is planting plans. Please help, I'm panicking at work. I'm the only designer who uses the program and we're a small company, this is crippling.