Zeebie:
I have noticed the same thing in my games. Sometimes one of them dies, and I lose them for years. Sometimes they're fine and nothing happens. I have a team that totals 293/300 (good is a total of 150, after that, its great) points, and for a total of 6 years, they found nothing on a large planet. I wasn't sure why, so I disbanded them and moved them to another planet, where they promptly found about 4 deposits in about three months. I know that the geological survey was not completed, but they just couldn't find anything.
I humbly suggest you try what I did and see if there is a change. I would be excited to hear of what your outcome is. I realize you've done this before, but even a system change might help, or changing the people comprising the team. Disbanding and putting them back together might help. It does seem that your team is fairly low. Your team (considering standard deviations) has a score of 93, so this is below par.
Your government type MAY have an effect on this as well. Ctrl-F2 might give you some insight that I do not have.