First, make sure he is fairly hungry, but not starving. If possible have someone other than mom give the bottle. If still having trouble, trying switching formulas.
If he isn't drinking it maybe it's because he doesn't like his formula. Try switching his formula to see if that is the problem. If it's not you should contact your infants doctor to see if there is a problem.
If you are just now intorducing formula to your five month old you may need to mix the formula with breast milk so that she has the same sweet taste. Try half and half at first and then gradualy go down to where she is getting 100% formula.