Healthy and active
Join Date: Mar 2007
Location: Caldwell, Idaho USA
Bikes: mid-60's Dunelt 10-speed, Specialized Allez Sport Tripple, Trek 7.2 FX
Sometimes little shreds of rubber in Slime get into the valve in strange ways that keep them from sealing or from allowing air in. I would guess your tubes have Schrader rather than Presta valves. When I have had these problems I introduce a few drops of water into the valve and work it back and forth. That usually clears the blockage. On Schrader valves I remove the core and use a Q-tip to swab the upper part of the stem.
I have had a love-hate relationship with Slime. After learning to use a few drops of water to clear the valve area, it has been mostly a "love" relationship. We have more than our share of goat's head thorns where I live. Tire liners stopped most, but not all of the goat's heads. But, they also destroyed several tires and tubes. I have removed them and am depending on Slime alone. I would advise not to give up on Slime yet.