I am analyzing data with the mental health component score (measured 0-100) for 4500 individuals. This score was measured at baseline and then 30 days following an intervention.
I would like to use regression to see the average change in score over time.

I used the following codes to reshape my data:

rename preop_mcs test1
rename x30d_mcs test2
reshape long test, i(sampleid) j(time)
xtset sampleid
xtreg test

I then conducted a mixed effects regression with the following code:

meglm test time || sampleid:

Then I added covariates:

meglm test time age sex smoking race bmi || sampleid:

I've noticed no matter how many or which covariates I've added the coefficient for time does not change. Because I know age is significantly associated with the score from my previous analysis, I'd expect to see at least a little change. This makes me think something is wrong with my coding and/or approach.

I appreciate any guidance with the above coding and how to proceed with a regression.