Are you sure you haven't priced the lease fee too high? A lot comes in to play when pricing a month by month lease, like quality of the horse, what sort of facility, if lessons are included in the lease, etc etc.
Despite living in a huge horse area with tons of showing opportunities for all levels, I always have a hard time finding a leaser. Part of it is that I am picky, and part of it is money on their end. I have one mare who is showing A circuit hunters/eq, been at 3'6"-3'9" so far and will go farther, however, I get a ton of interest from lower level folk not wanting to show (nothing wrong with that!!), but when I have posted in the add specifically that leaser must be willing to show on the A circuit, it's a little bothersome. That, or I do find someone I really like, and then they ask me to lower my prices significantly or the notorious "so what's the lowest you'll go!?". My lease price really isn't that ridiculous or unheard of, especially considering her quality and level, and the barn I am at so I am actually a little shocked how many people ask me to lower it when it's actually quite reasonable.
My advice would be to take those things into consideration, as well as the terms of your lease. For example, if your board is $500 per month and you want to lease your horse out 2-3 days per week, don't charge $400 for the lease. I always base my lease prices off amount of days per month the leaser would be riding. Let's say it's a 3 day a week lease (that includes their lesson day), my board and farrier each month for 1 horse is $720, so 3 rides per week equals 12 rides per month - so $720 divide by 30 days $24 per day, times 12 days is $288 per month, which is actually quite low for what would be considered a half lease around here, most people would bump that price to anywhere from $350-$550 for a half lease in my area.