missing observation after regression

[Zorodzai Murinye]

I have a balanced panel dataset with 4,833 observations. When I regress y on all x1 x2 x3 and x4 my results show n=4,833. When I regress y on x1, n reduces to 3,420 and regressing y on x2, I get n=4,238. What could be the reason for this? I am using areg command in stata.

[Joseph Coveney]

What could be the reason for this?

areg whatever x1, absorb(id)
generate byte x1_only = e(sample)

preserve
keep if !x1_only

list in 1/20, noobs sepby(id)

restore

// Repeat for x2_only

[Zorodzai Murinye]

Thanks Joseph