Leaking Slime inner tube valve after opened for first time since install
You might want to remove your Schrader valve's stem (innards) with a stem removal tool and check the valve's rubber seat, located on the part that you remove. There may be debris lodged in the valve seat such as grit or sand or some dried slime that would cause it not to seat and seal properly. Also consider switching the valve stem core from another valve from say an old inner tube and see if that does the trick. Unless the valve was damaged or the old tube you take it from is real old, the valve stem core will usually last much longer than the actual tube. Many years ago when my bike had Schraders, I'd keep a couple of valve cores from old tubes in my patch kit in case I experienced a valve problem when out on a trek.