> As an employer however, we've found it hard to always keep our developers motivated and executing on time.

The only time I've seen this to be a "problem" is when one of the following is true:

A) Non-technical people are responsible for the estimates and/or have the ability to make changes to the requirements without understanding the complexity of the new requirements.

B) They are stuck fixing bugs instead of doing new feature development due to technical debt.

C) There is a serious communication problem.

D) People want fixed, written in stone deadlines before the prototyping is done when things are still too fluid for anything resembling a good time estimate is possible.

For instance, the only deadline I've majorly missed at my present workplace is the one project I told my boss was impractical, a terrible idea, and to please give it to someone else because I don't have the temperament to handle such requests. I'm sure some "business" person responsible for it considers it a "motivation and executing on time" problem despite the fact I basically said this was a terrible idea in front of the entire IT department in an official meeting with minutes being taken.

Are you completely certain none of these 4 exist as an employer?