Having played around in the ME a bit, it seems, for this current version of DCS, successful AI F-14 tanking is a function of both the mass of the F-14 and its speed. A more heavily laden F-14 will require the tanker to be travelling faster for it to successfully plug the drogue. Finding this right speed is a bit of trial and error, which is annoying and time consuming. For reference, a heavy'ish F-14 at 20 000ft will need the tanker to be flying at least 360knots (as set in the ME).
Rokkett's post in AI F-14 Unable to Successfully Refuel from S-3b tanker was marked as the answer