I use something called UltraSeal for the really tough sealant jobs and love that stuff.
Ive tried various sealants and I think the Green Slime has a good reputation around here, too. We have locust thorns whick really take a toll on our tires.
While researching sealants, one sealant supplier told me, as I was complaining about a sealant that did NOT work, she informed me that on a low pressure tire, like an ATV tire, the tire needs to be pressurized up to the bead seating pressure for a little while to push the "GOOP" into the tire wound. Then I should release the air pressure to normal psi, which is 3.5 psi for my four wheeler.
After I took her advice I have had great success with tire sealants-even in low pressure tires.
OBTW, with a high pressure tire, like a boat or small trailer tire, the normal pressure is around 50-60 psi, and therefore, you get good results right off the bat.
something to think about.
dwight