How to precipitate Chloroplatinic acid?
I suspect that you could be forming an aqueous solution of $\ce{(NH4)2PtCl6}$.

I would like to make an observation as a person who has done platinum group metal chemistry, the anionic chloro complexes of PGMs like $\ce{PdCl4^2-}$ and $\ce{PtCl4^2-}$ are very toxic. They can induce a nasty allergy to PGMs. I would suggest that you do not work at home with these substances.

Also in my experience at least with $\ce{PdCl4^2-}$, I have always found that if you want to bring it out of solution then cation methathesis with a potassium salt can bring the substance out of solution as a brown solid. I suspect for the hobby chemist that low sodium salt would be the best reagent for doing this transformation.

For the lab chemist I would suggest using tetramethyl ammonium chloride or even tetraphenyl phosphonium chloride