I did tried this alkali precipitation method but it did not worked out on Pt.
However this method is very well workable when using Pd and Rh
At least this publication in my link and 3 patents (found in google patents) use alkali precipitation method with success.
Try to change some parameters, eg temperature, time, concentration and excess of nitric acid (I understand that the problems are at this stage of the synthesis).
The alkali precipitation method will remove chlorides almost completely if you manage to master them.
Fractional crystallization (suggested by Jorge Stolfi
) is extremely labor-intensive (especially on a small scale) and not always effective due to the possibility of forming co-crystals (we do not know all the properties of platinum(IV) nitrate except its good solubility in water).
How did you check that the method you describe allows you to get platinum(IV) nitrate? It is used from time immemorial (over 90 years) for PtO2
synthesis without isolation of nitrate.