If the nappy gapes around the legs or has loose bits and the elastic doesnt spring when pulled then its the elastic but if the nappy is wicking rather than leaking then its more likely to be to do with the inserts, if they are bunched up or dont meet the sides of the nappy inside then the moisture will just seep out the actual nappy shell.
Check the fitting compared to the others, if they are the same nappy they shouldnt be different, if they are you should contact the seller.
As probably said before mf is naf, it squidges the moisture out easily and soaks quickly.
We have vest extenders and sometimes even these are still too tight with some nappies, if the other clothes (espcially jeans) are too tight then you will have the same problem, vest extender or no vest extender.
Hope that helps
Oh and there are people on here that may replace the elastic if you post a 'wanted' or in the wahm section, but if you have only just got the nappies this is still something that you should take up with the seller, sometimes people dont realise the state of the nappy especially if they never owned one new to compare. x