Was it the same job today as the one you upgraded yesterday? It sounds like you're talking about a featured job, where the upgrade to unlimited invites only applies to that one job.
It looks like you have two job posts, one of those was promoted to Featured Job for a one-time fee of $29.99 and you received unlimited invites for that one. The other job post is the regular one and therefore had 15 invites available.
Let us know if you need any further assistance.