There are several things you can do to get the players interested. The most important one, of course, is to ask the players why they felt bored, and what they were expecting. Communication between GM and players is the key ingredient to a fun time to be had by all. Talk to your players; let them know that you are trying, but want to get their input as to why they feel bored and unhappy. This will give you some insight into what to do better next time.
The next thing is to use things in your campaign world against the players to get them thinking about stuff. Such as:
1. The plant is used as a weapon, right? What kind of weapon? And can the players be assaulted by a rogue person with one of these weapons as they are trying to get to and/or on the transport off that rock? And darn it all if being attacked didn't make them miss the transport!
2. When ingested, the plant makes someone euphoric and open to suggestion? PCs have to eat and drink at some point, so why not have someone slip a little of the plant into a PCs drink? Then you get to tell the player that he is feeling wonderful, and that he feels more open to suggestions. Make sure it's a PC whose personality isn't always happy-go-lucky, and that will get the rest of the group asking why he's acting so weird. If it happens to more than 1 PC, that's all the better because then they will truly know something is going on.
3. The reverse of that would be to have the NPC they are taking home come to his sense before they get to the transport and tell the players his story about being on the planet, and that he thinks he may have been drugged or something. Once they see that his mood changes dramatically, they will start asking questions.
These are just suggestions, of course.
