its happening simply because there is no snap holding up the sides of the front. Their are benifits to haveing two snaps & to having 3. 2 snaps will give you longer inthe nappy as in bigger sizing but 3 will hold up the sides preventing wing drop.

When I did Isla's I thought it was crazy they would only do 2 snaps but now she had a growth spurt around the belly & I can see why only 2 is better once they get to the otter snaps in sizing.
But if you think DD will be in the smaller snap setting for a while then send them on to me & I will add the third snap for you.