Don't confuse being tracked with being displayed. There are a bunch of reasons why a blip may not be showing, even with fully serviceable equipment.
e.g. No bearing 'other' traffic, Interference limiting mode, TA or RA underway. Or if its not as close as you think it is and isn't actually in the display volume.
As a result it is quite routine to see blips appearing and disappearing and should not be taken as the TCAS being unreliable or used as a justification for ignoring an RA.
pb