Few things of note; if you're a Super Recruiter, or the referrals in question took place on accounts which previously had an active SIM associated with them, then you won't get Payback for them.
If not, and since the referrals were within the last month as you state; then any which were during December (if at all), the points for those should be visible now in your Payback points tally, and any which happened this month will show up in it next month.
In turn, Payback is paid out twice a year in both June and December, so as such we're still fresh in a new period, so you have a good few months to garner some more points.
As I mentioned above regarding Super Recruiters; Payback for recruiting new members is capped at 15 recruits. Once you hit this figure you will no longer receive Payback for SIM activations and are enrolled onto the Super Recruiter programme where you're still able to earn money, though it's really for those serious about recruiting.
Do keep in mind that you won't receive the mentioned credit if you're a Super Recruiter, or if any SIMs are activated on accounts that previously had a SIM active on them which expired (i.e; returning members).