It's not uncommon for weight gain to be slow for the first few days, specially if you take into account that when you weighed them at birth they may still have been a little "wet" from the birth fluids which makes them seem slightly heavier than they actually were. If they are feeding well & all are warm & look nicely rounded, not scrawny or cold to the touch, then they are probably ok. You just need to monitor them carefully for the next few days. Not all puppies double their birth weight in the first few week but you would definitely want them to have put on a few ounces before the end of this week.