What Rumpty said but I'll add....
If you're using a standard blade then your blade tension may not be high
enough. Haven't seen a scale yet on a Jet or Delta BS that is anywhere near
accurate. Best advice, keep adding tension and test again. I have a 1/4"
blade I use once in awhile and the tension is double what the scale
indicates for a 1/4" blade.
If it's a low-tension blade, like a Suffolk, measure the horizontal
deflection as per their instructions. For a 3/4" wide blade, I believe they
say 1/2" deflection max at the center point but I don't have the
instructions here with me.
Also a 3tpi or a skip-tooth blade is recommended for resawing. More teeth
than that and the blade will wander since the gullets get filled and the
waste gets hot and gummy and it forces the blade to deflect (bow out).
Also, slow down so the blade has time to clear the gullets and stay cooler
if you don't have a resaw blade.