Stabilization of Methylsalicylate

Respected all,
Can anybody guide me that how to stabilize the Methylsalicylate in Gel formulation of diclofenac diethylamine, upon near expiry it fails to pass the test for its assay.
Upon our aspect, it may undergo transesterification process, if it means there should be an initiator like acid or base, but in our formulation, we don’t add any acid/alkali excipient so it wasn’t possible for the reaction to take place, so help us to solve this.

good question you can use some stabilizer and try and even search some patent you will get ideas. thankyou.