Hi Emma, fixed price projects can be a minefield, for sure.
I always make sure that all requirements are clearly agreed upfront. Discuss a brief with the client and ensure they’ve confirmed it before you accept the offer. Specify how many revisions are included in the price. Make it clear that any redesigns (i.e., changes to the brief) and additional revisions will cost extra and require them to fund a new milestone. If there’s anything else you need to be in there from the start for your kind of projects, you’ll learn that along the way and can also include it.
Hope that helps. In terms of the specific projects you mentioned, it seems like the first client has basically admitted they made a mistake so they may be co-operative in terms of negotiating a new milestone to cover the additional work. If not, you’ll have to decide whether you want to carry on until the job is complete or cut your losses, give a refund, and walk away.
The second client seems like it could be a case of ‘scope creep’, although it depends how clearly defined the original brief was. Either way, if they’re not communicating, you’ve done the work and maybe they’re happy with the latest version and they’ve moved on. Submit the work against the milestone (if you haven’t already) and claim payment.