Distribution. Online sales and distribution wasn’t an issue but to generate an increased product awareness you really need those books out in (physical)book stores and out in quantity. Yes you can get books stocked in stores such as Waterstones and independent book stores but without a decent distributor you will struggle to get a decent quantity dispatched at any one time. You can bypass this hurdle by paying distributors or even self publishing print companies for the privilege but the added (extortionate) cost tends to be too high resulting in a net loss per unit sold.
Having limited distribution hurt me the most when after a prolonged marketing campaign demand increased to the point that I couldn’t match it. I had the books but not the ability to get them stocked in shelves. Furthermore I could not take up offers of book signings where book stores typically order 60 units on a sale or return basis to be signed and sold throughout the book signing event.
On a final note I did think it was a beautiful gesture on Waterstones behalf that they do allow self published authors to stock with them – giving small fish (like me!) the chance to swim with mainstream authors. (Waterstones book seller and junior club book reviews here.)