First question: Which difficulty are you playing?
If it's NM or HM, it's very unlikely to max out the level of 14 units. I normally can max out ~7-8 characters in these difficulties.
Elincia starts at level 1 very late, so you have to do stave abusing with her (physics, fortify, hammerne, rescue).
Astrid has paragon (=doubled experience), so it's easy to max out her level.
This thread includes a list about the units, who should be brought to maximum level in PoR.
Even if it's made for FE10's hard mode, most of these transfer boost suggestions also apply to lower difficulties.