It sounds like you're in the price discovery phase of the business. If you can't get people to convert there's a few questions you need to ask... 1. Is the price too high? If you truly only need to scan once a quarter or once a year, it's going to be hard to ask for a monthly charge. 2. Sites are different sizes so do you charge a flat fee regardless of how big the site your scanning is? Would seem unfair if I was a tiny site and I was subsidizing larger sites because we're charged the same rate. 3. How efficient is your checkout process? Is there any friction you can remove?

Out of your potential avenues 1 seems most probable for higher revenue and what you should do.

I would ask the question why am I charging a monthly fee. Personally, your service sounds like you need more a credit system. Buy credits (could correspond to pages scanned, or just raw executions), with a minimum number of credits ($10 = 10 credits), and allow people to setup schedules to scan when they see fit. Maybe you'll see people mostly schedule once a quarter. You can have credits automatically top-up when it reaches a certain threshold (ie 5 credits).

Hope that helps, and good luck!