The most common mistakes are:
1. LED strip is connected from the back side: addressable LED strips have a direction indicated by small arrows on the strip. The data line must be connected from the side where the arrow begins.

2. Incorrect GPIO set: Check in WLED settings whether the correct GPIO number is set as data output. The ready-to-use controllers usually have the correct settings printed either on the housing or on the circuit board.

3. LED strips with many LEDs connected and set, but the current limiter is still at the default value (850 mA). This often has the effect that LEDs flash briefly when the color changes and then go out immediately or light up very dimly. Check and, if necessary, correct the settings in WLED.

4. Wrong LED type is set. The correct LED type and the correct color sequence (RGB, BGR, etc.) must be set in WLED settings. Check that these settings are correct. You have to know the LED type, but you can simply try out the color sequence to see which is the right one. Special attention must be paid to the WS2814 LED strip, which must be set as SK6812 in the preferences.

5. Wiring is bad. Wiring must be done thoroughly. Loose contacts, cold solder joints, etc. must be avoided. You also need to be careful about the correct sizing of the cables for safety reasons.