Clay is absolutely correct, avoid neutral or basic pH balance at all costs.

When I mix, I add the oxalic acid and then get it all into solution. I then add a little extra until the pH is down below 6. A little goes a long way on this. Check the pH with test strips from Edmunds Scientific (about $1.00 for 100 strips).

I think that the problem is that oxalic acid can be hydrated, or it could be somewhat dehydrated (it will hold 2 water molecules per oxalic molecule). If the oxalic is not fully hydrated, it will be more potent per given mass, and vice versa.

There's absolutely no concern about ensuring proper pH by adding more oxalic initially, and I recommend you do that as a matter of course. It also makes sense to check the pH of your developer solutions every once and a while to ensure that they haven't drifted too much. If they have, add oxalic and test again.