Hello there. Sorry to hear about you challenge which is common and very frustrating. I've worked with numerous organisations large and small (including several web/marketing agencies) over the last decade to help make their projects more successful.

I would expect that there may be some tweaks you might like to make to help your project workflows, e.g. building some agility into the execution phase of the project. These may provide more flexibility and effectiveness, but I suspect that your challenge will be more readily fixed by building the reality of projects into your client engagement. Do you discuss upfront the risks to meeting their deadlines and scope from from delay on their part? Could you adjust the payment milestones to protect you from "the inevitable" or incentivise the client to respond promptly?

I've not seen any specific web design pm training courses, but there are lots of very good 1 day introductory courses out there.

Alternatively if time and budget are an issue I can offer a personal 90 minute interactive online PM Foundations session which provides basic definitions, terminologies and best practice sprinkled with lots of common sense ideas? Feel free to get in touch if you'd like to explore any of the above some more.

Good luck!