If the grid is not accurate to the song, you either have a beatgrid whose size is incorrect, or a song whose tempo is inconsistent. There are two fixes.
If the grid is the wrong size, use the buttons to stretch out the beatgrid or compress the grid so the markers are closer together. Take note that any small adjustment at the start of the song will make a big difference at the end of the song, so jump around the song.
Second, if the song beat is inconsistent, like when it has a live drummer, you should use DYNAMIC analysis mode, and even then you may need to make adjustments to the grid. See this video for making adjustments to the beatgrid in a song with a large tempo change - you may find some hints there to tweak your songs too.