Glad to see other people had trouble with this and not just me! I did work out a pretty complicated workaround. It seems tricky but once you get it then it all works VERY smoothly and I now miss very few subIDs.
There are a few important steps here. If anyone can tell me a simpler way of doing it PLEASE tell me!
1) Firstly you MUST have an initial redirect (META redirect, not HEADER) that looks like this:
index.php (with prosper cookie)
-->
lander (with aweber form)
You MUST use this redirect or the whole thing won't work. For some reason (I don't know why) the tracking code that is set by the prosper cookie will not pass to the aweber form unless you redirect first.
2) On the aweber form, use aweber's meta_adtracking to pass the prosper tracking ID into aweber's optin database.
Remember this code is the tracking ID. NOT the subID yet.
3) Send your subscribers to a redirect from your followups using a link something like this:
www.yourdomain.com/offer.php?tracking={!ad_tracking}
This should pass the tracking ID to the prosper redirect which will covert it to a subID. BUT instead of using a $_COOKIE you MUST use $_GET.
For example, instead of this:
&pci='.$_COOKIE['tracking202pci'];
use this code:
&pci='.$_GET['tracking'];
And this is it! It SHOULD work if you follow these steps. Let me know if you need help getting this to work :)