Assay of Capsaicin

Respected all,
Can anyone provide me with the method of analysis for Capsaicin in liniment, we have followed USP monograph for its analysis - what we face is, at initial we get good result & peak was also good. But upon expiry time, the peak was getting merged.
We have changed the ratio of mobile phase, oven temperature etc, what it reflect is, the Sample elutes well but the standard doesn’t and vice versa.
So, help us with the same (we have already referred EP, JP, BP etc., other than that will be more helpful for us).