A removed air valve leaves round marks on top fabric, try to match same, does it have more than 1 round mark, if so, probably last owner must have removed valve and now doesn't sit properly releasing air. Check valve's below has intact the plastic protruding ring that seats & seals against tube's fabric.
With soappy water, moisten upper tube fabric and below of valve, will slide much softer till a complete stop. Air valves must be seated tight with tool, not that tight as to break valve though...
Happy Boating
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