Well is this a reinstall of the old belt or a new belt? If new belt did you get correct one as there is three different belts depending on the deck spindle pulleys and serial number.
The spindle pulleys that are 6.3" uses the A x 118.25 belt. If you have 5.75 spindle pulleys, it depends on the serial number if you are needing the A x 122 (SN 1A275H and before) or the A x 122.5 belt (after 1A275H).
And you must disconnect the idler spring in order to get the belt on and then reconnect the spring afterwards.