Please tell me which agency is it.
Also, if I understood you correctly, the problem is "Checking unfinished list" never stops ?

Please also specify how many images you upload at once and how many images are there already on the site.

There is no limits set in the software.
And I didn't heard of any limits on 123 or Pond5.

Please describe the problem with Pond5 and 123 more detailed.
What exactly happens when you try to upload images? What statuses are appearing to the right from the agency name? For how long?
Does it stuck at some status? Does it just reset timer to 15 minutes without the submission?
In any case, please launch submission to the single agency at a time (without parallel submissions or uploads), let it either reset timer to 15 minutes (if it does so) or hang for 2-3 minutes (if it is stuck at some status) and then send report via Service - Report an issue menu command. After that please post here and I'll check it out.