Soldering problem when changing component on tablet board
The diode may be connected at one (or even both) sides to large copper masses in the form of a ground or power plane.  Consequently it will take a lot of heating to melt the solder.  Not higher temperature, just applying heat for longer.

In this situation it is often good to pre-heat the whole board, either on a soldering hotplate, or in a reflow oven.

The other possibility is that the diode is on the "underside" of the board and may have been glued in place before soldering to hold it in place.