Ammonium molybdate tetrahydrate is soluble in water. You may have loss of ammonia and molybdic acid has low solubility in water. Try to ad 10% ammonia to it.
Dissolving of ammonium molybdate in water results in formation of different species including polyoxomolybdates. The distribution of these species is pH and concentration dependent. You may increase the solubility, but the question is what in solution.