I know in my 75 (same tank as yours) it will read near empty when there is still a quarter tank left. I'll then fill it and it'll only take 15 or 16 gallons (not 21) to fill it till it's spilling out. So you could be too low to read.
Try sticking some siphon hose in the tank and see of that contacts the float and alters the reading at all. Otherwise add another 5 gallons.
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