Ferric oxalate goes into solution, but takes quite some time to do so - a day or so is normal. You have to shake it once in a while, or zip in in the microwave for 10 seconds or so several -well, many - times, shake, zip again. Take care it does not get too hot.
Leave the cap loose, of course, or otherwise it will blast off again as you described in your other thread - which I saw too late, sorry, and will try to answer here.
Ferric oxalate is amber coloured when it does not contain free oxalic acid, and gets greener the more oxalic acid is inside. Personally, for pl/pd prints I prefer ferric oxalate with considerable amounts of oxalic acid (the exact amount is not crucial), and some EDTA tetrasodium, because prints printed in this way clear very much easier (I seem to remember having read once that too much EDTA may cause bleeding or some similar problem, but I never experienced this myself).
For kallitypes on the other hand, you want the amber colour, for the acid - presumably also other additions - causes the silver nitrate to precipitate.
However, I once tried to get ferric oxalate here in Germany, and could not get the amber-coloured stuff into solution, try as i might, wondered what was the matter, until I noted that due to a translation mistake - easy with such comparatively unknown substances - I had got ferrous oxalate (iron III oxalate, whereas ferric oxalate is iron II oxalate). Ferric oxalate turns into ferrous when it goes bad.
So you should make sure what exactly you have got. But if it is ferrous, don't despair:
My mistake turned out to be a happy accident, as it is quite easy to make ferric from ferrous oxalate: put the stuff into water add concentrated oxalic acid (made by heating the water), then add with a dropper hydrogen peroxide. A reaction will start, shake, drop again, add some more acid if necessary, or a little more ferrous oxalate if the stuff gets unwantedly greenish, after a while you have it all in solution. Give it a rest with a loose cap, as it continues to produce gas for a day or so, but then it is ready for use. I now prefer this method from bringing powdered ferric into solution.
The - necessary - test to know all ferrous is turned into ferric: put a drop into a weak sol. of Potassium hexacyanoferride: the drop must not turn blue. This is also a good test to see whether old FO is still good, provided no EDTA is inside, which makes the test useless. But rather than pouring old FO away, with a few drops of peroxide you can bring it to full spark again.
For your potassium oxalate developer: I have got the proportions somewhere, must look for them. But something must have been wrong with your testing: adding oxalic acid must bring the PH down, and what you want is neutral or slightly acidic.
