Band saw blade is out of alignment.
The teeth of the blade need to be riding the crown of the wheels, and the blade guides should not really make much contact with the blade unless you are cutting.
Look up youtube videos on band saw blade alignment.
2 things:
1. Until I learned this - "teeth of the blade riding the crown of the wheel," as Skie mentioned above, I could make my 14 in BS go straight but I had to often make adjustments. When I learned to begin alignment with the teeth on the crown followed by other alignment settings, I have been amazed at the straight cutting on various grains. And I have not had to have as much blade tension as before.
2. SOME 10 inch saws don't obey number 1. I have a 10 year old 10 inch Delta. I put new guides, bought new blades, aligned and aligned and aligned. Very little straight cuts on 3/4 inch stock. I spent hours and hours on it - just trying to get it to cut straight.
In my opinion, Some 10 inch BS's are not meant to deliver 3/4 stock on par of a 12 or 14 inch. They are more on the line of hobby saws for 1/4 and 1/2 at the most. There are 10 inch BSs that do deliver with 3/4 stock consistently.