Degradation of L-Ascorbic acid in the formulation is resulting in the degradation of Triprolidine HCL,which is one of the active ingredients,hence the need to come up with a stable formulation.
I recommend lowering the pH at 3.2 with a buffer using ascorbic acid/ascorbate system, and protect from light; Triprolidine is light sensitive and is pH dependent.
Triprolidine HCl is sensitive to light and high pH. I recommend you to use an acidic pH with appropriate buffer capacity or formulate your system in suspension form. You can also exploit microencapsulation to protect the active ingredients from deleterious environmental factors.