This is a hard one without seeing the lame dove, but it sounds more serious than just slow development. Do her legs and feet look the way they should though weak? As in well formed and straight bonewise, toes spread nicely and it seems to be a muscular issue? 

 

Or are the feet or a foot curled or the legs at a strange slant.  Sometimes poorly nourished young dove's legs take on the curve of the "nest".  I have a non-releasable dove who has that problem, her feet are so curled that she cannot sit a perch though I have read that little cardboard "braces" have now been used to help the doves with this issue if put on them young enough.  There are also some viruses which can cause paralysis of the legs.