You perhaps have used the best stuff for DNA isolation here. I think all you need to do is to pool as much DNA as u can and concentrate using evaporator or absolute ethanol/3 M NaCl method

Concentrating the DNA will not get me more DNA though, just higher concentrations. I need at least 2 ug. I have, though, tried some kits to concentrate the DNA but I lose too much of the DNA in the end.

In case it helps, this is mainly for amniotic fluid samples.