Yes that is called DRIFT,  it happens on many scales and is often a reflection of temperature changes or instability.

If you read scale manuals from Sartorious, Mettler, My Weigh or others you will see they all say the same thing in one form or another:

Remove the weight immediately after a stable reading displays on the screen.  There are two reasons for that, the first is that the longer you leave an item on the load cell the more it shortens the life of the load cell.

Load cells are basicially hollowed out blocks of special aluminum, when you place on object on the pan the load cell BENDS - and a sensor detects how much it is bending thus changing it's signal (then though various electronic circuitry you end up with a weight reading on your display).  The more times you 'bend' the load cell through weighing, the shoter it will last (eventually it will become 'bent' and will need to be replaced).  If you weigh items and then remove them immediately the load cell could last for tens of thousands of weighings.  If you leave items on the tray for long periods of time you are shortening the lifespan of that load cell DRAMATICALLY!

Anyway to compensate for negative drift please either check the manual for adjusting the auto zero function (turn it up) or try a warmer room,  try turning the scale on and letting it warm up for 3 minutes before use, and check the scale with an external level to be sure it's totally flat and level.   Last but not least try a more stable surface such as a neopreme coated table (mousepad will do) and check for vibrations or electronic interference in the room.

Have I said enough?  Sorry for the ramble but your question is common and I hope other readers of digital scales.com / Scale Magazine.com will have their scale questions answered by this post.