@SleazySue Son't worry about the delay; people troubleshoot when they have time.
Start by deleting the five cache files in your Sims 3 game folder in Documents\Electronic Arts. These files should be deleted regularly anyway, as they tend to clog up with stale data. While you're troubleshooting, be sure to delete them every time you quit the game. For reference, they are:
- CASPartCache.package
- compositorCache.package
- scriptCache.package
- simCompositorCache.package
- socialCache.package
Load your save and completely empty the inventory of the sim who can't plan the plasma seeds. (If there's something you can't remove, don't stress about it too much, but get everything else.) If that doesn't make a difference, go to Edit Town, split the one sim from the rest of the household, evict just that one sim, and put them back. As long as you do this without leaving Edit Town, the sim will keep their job and relationships but lose any promised wishes and opportunities. Then use "save as" to rename the save (so you have a backup), quit the game, clear caches again, reload, and test.
If that doesn't help either, go to Edit Town and split off the one sim, but this time, move them to another house. "Save as," quit, delete the cache files, reload, and move the sim back in again; it's fine to use Edit Town to merge the households rather than the usual social interactions. Save, quit, clear caches, reload, and test.