Radar Screens and Aircraft 'blips'

[sharpclassic]

Hello all,

Question from a pilot to all you controllers.....

Are there any circumstances under which a controller may not see the 'blip' of an aircraft on his/her screen?

e.g. 2 aircraft cruising in the airspace of a large European nation, both at the same FL on reciprocal headings in the same airway.

Thanks very much!

SC