Sodium lactate may have minute amounts of lactic acid present in the 60% solution you have. That free lactic acid reacts with NaOH and produces more sodium lactate, therefore crystals on the bottom. In addition if you heavily discounted water since milk was to be added, then you had a supersaturated solution of sodium lactate.
Best way to add lactate is after all NaOH is disolved.
C3H603 + NaOH --> H20 + NaC3H5O3 Lactic Acid + Sodium Hydroxide combines to form Water + Sodium Lactate.
Thanks velvet, awesome answer!