By the way, my springs   change in force ,  but they still raise or hold the hatch above head height and enough to provide a good rainshield for the person cooking. The force should be temperature dependent (lower temperature = lower gas pressure), I will observe to confirm this hyphypothesis.  Reference the gas law from your high schook chemistry. In  new springs with good seals and smooth unscratched pistons, this is the most likely explanation.