[mystreet123]

Why sometimes the two lines of a doublet have slightly different intensities?

[ATMyller]

Bad shimming, phasing, or baseline is not level.

[Babcock_Hall]

When two nuclei are coupled and the difference in chemical shift is on the same order of magnitude as the coupling constant, the intensities of the inner peaks increase slightly, relative to the two outer peaks.  This is sometimes described as the doublets leaning toward each other.  My impression is that a proper explanation of the effect would have to use quantum mechanics.
https://www.chem.wisc.edu/areas/reich/nmr/05-hmr-09-2ndorder.htm

[Babcock_Hall]

Occasionally one encounters a doublet where the two peaks have the same areas, but one member is sharp and narrow, and the other one is short and broad.  This has to do with chemical shift anisotropy.