We have seen this before and we are working on correcting it.
In the meantime, you should be able to add the Contact even to lists which are not listed on the Sign Up Form. You need to include the OptInSource tag **in the Contact tags** (not just in the Contact lists tags). This means the middle of your entry XML entry might look something like this:
...
<CustomField14>some data</CustomField14>
<CustomField15>some more data</CustomField15>
<OptInSource>ACTION_BY_CUSTOMER</OptInSource>
<ContactLists>
<ContactList id="http://api.constantcontact.com/ws/customers/ACCOUNTNAME/lists/10">
<link xmlns="http://www.w3.org/2005/Atom" href="http://api.constantcontact.com/ws/customers/ACCOUNTNAME/lists/10" rel="self" />
<OptInSource>ACTION_BY_CUSTOMER</OptInSource>
<OptInTime>2008-06-23T06:46:30.000Z</OptInTime>
</ContactList>
<ContactList ...
Thanks,
Tom M
Group Product Manager – Content Editing
Constant Contact