Proximity sensor changes accuracy over just a couple of days

Hi, I wonder if anybody has experienced such a thing with these sensors.
Recently I installed a proximity sensor for bed leveling - those PNP NPN ones from eBay. I configured it, it was working great, but after a few days I had to reconfigure its distance because it started to trigger from a shorter distance.
I thought it was a bug in me so I moved on.

But after a few days that happened again - and now it triggers from such a short distance my hotend touches the bed before the sensor activates.
I wonder if it's worth bothering anymore with this... Is this a standard thing that happens with these sensors or it's just that I bought a buggy one?

have you experienced such a thing?

Edited 1 time(s). Last edit at 05/05/2017 02:32PM by ltomov.

Is it inductive or capacitive? The capacitive ones in particular are affected by temperature and humidity.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for the reply, it's inductive.