im not a breeder and never had joeys.  BUT from reading from different sources it 
sounds like it is not all uncommon, specially with first time moms. 
   I feel like some of the issues that could make it harder is how safe the glider 
feels.  New home, or not bonded or friendly with owner.  Also a really big issue 
IMO is diet.  with critterlove they have a breeders formula and with OHPW they  
request a DOUBLE portion of wombaroo high protein powder in it.   I just get the  
feeling many owners don't take this into account.  the strain of nursing takes  
a lot from a mother.  without the extra nutrients she just cant handle the nursing. 
in the wild they can hunt for food longer, in a cage they are stuck with what they 
are given.