this does not happen often with babies born on term (37-42 weeks)!
they shouldn't lose more 10% of birthweight, and should get back to it within 3-10 DAYS! some breast fed babies migt take up to 3 weeks, but anything beyond that is a problem.
I'd be VERY concerned!
First I would get baby checked for tongue-tie (TT) - failure to latch on could be caused by this easily operable problem!
my kids had this problem, DS3 was 9lb 6oz, went down to 2nd centile by 3 months and not only took 4 weeks to get back to birth w, but gained so slowly it was awful... I didn't know he had TT as I never heard of the thing and I only discovered it when he was 8 months old!
DS5 also took over a month, though he was a big lad at 9lb 14 oz. I noticed his TT straightaway, it was sorted at 3 weeks old
DD took 6 weeks, it was a big thing and she had to be checked by hospital pediatrician. she was 8lb 3oz at birth, she too had TT, fixed at 3 weeks
out of our 6 kids only DS2 was a fab feeder, so I suspect that DS1 and DS4 also had it to a certain degree - never checked....
if tongue-tie is not a problem ther has to be something else that causes "failure to thrive".
get that baby checked out asap!