Can a detector erase a signal?  NO.  It can't 'erase' anything.
A detector might produce a dig-worthy signal if a target is swept one direction and not on a reverse sweep.  That might be caused by the orientation and the ground environment, a nearby rock or intense "mineral body", and most often by a nearby masking trash target.
Of course we hope for a more ideal condition where we get a good signal if it is swept in each direction from left and from right.
And YES, there are times we get a good response or two, but on repeat sweeps the target signal weakens or even vanishes.  Why?
Several reasons possible such as not a direct sweep over the target.  A weaker deep signal and the ground mineral environment.  And really the 'unknown' with regard to Ground Balance.
How?  It could be 'off' a little or it changed a little.  What causes the change?  More detectors than most folks realize rely on a GB for the Disc. mode that relies on Tracking or is a fixed or preset GB internally in circuitry that might shift (track) just a little from reading the ground on repeated sweeps.
Even models with Ground Grab / Fast Grab or Manual GB often have that adjustment for the All Metal mode or Pinpoint function, but it doesn't carry over fully for the Disc. mode.  Maybe a little within a defined range, or fully, but generally not at all or not a full-range adjustment in the Disc. mode.
What to do?  Simple:
Do not make multiple sweeps over a good target.
One or two decent hits just quickly Pinpoint and recover the target.
Monte