I think as previously stated once we reach a limit in our customer base,a limit that we think that we have "arrived" at where we want to be then simply there seems to be no more challenge.So move onto the next challenge....
Diversify your business,add on other types of services that your customers might require...it may be gutter cleaning,upvc cleaning,pressure washing,gardening,painting & decorating.
BUT of course if you have the time to add these to your existing business it does mean that you have not built your original business to its maximum potential,unless you are employing or sub-contracting work out.
Window cleaning is a repetative job,but so is stacking shelves or filling out forms sat behind a desk.Look at your strengths in your business and work on those,if you are bored with certain aspects then delegate them to someone who can help you in that area.
Just my thoughts,but i know where my bread is buttered,bored or not
