I have the same problem just one girl, always weak or extremely weak.  I've covered all the options in the store, including the dr. visit, and the stronger injection - even though the doctor didn't say she needed it.  But she's always sick since she was about 35 and now she's 50-something.  I'm afraid she's not going to make it much further. She's a darling little thing too.  I'll miss her.