No, can't really lose it in the line.
A leak perhaps? Has the keg been under pressure all that time, and it still is?
You get any foaming?
On a sidenote, I brewed a 2% Oatmeal Mild that fails to carbonate under 15 psi, flat as can be. I was able to carbonate it under 40psi, pours like soda (line is too short, balanced for 15 psi), with a big bubbly head that dissipates within 10 seconds, leaving no foam. The beer tastes carbonated, but not much. I guess there's a lack of proteins in it to form a head, or something else wonky with 40% oats in it.