if the area you select is to large it loses quality during the JPG compression. i found the best way to create a custom map is to decide on wat zoom level u want. then enable the grid level two zoom levels higher.
eg is u want zoom level 14 maps use grip 12.
create an individual tile for each square of the grid. remember there is a maximum of 100 tiles
it can be tedious to do a large area but this method ensures the maps are highest possible quality