Troubleshooting failure (low output) in LED tube bulb
The marked components to the left are resistors.  &quot;102&quot; means 1 kiloohm. Remove them from the board and check the resistance.  They look OK, though.
There are two diodes, D1 and D2.  Remove them from the board and test for diode function using the diode test function on a multimeter.  Should show around 0.7 V in one direction, open circuit in the other.  They might have failed open circuit.  If they aren't marked for polarity, add a mark to them so that you get them back in the circuit with the same polarity.
The unmarked gray and brown SMD parts are capacitors.  Remove from the board and check for short circuits or open.  You might also measure the capacitance.  You have several tubes, see if the capacitors all show about the same capacitance (compare capacitors marked C1 to C1 from another light.)
One LED in your photo looks different.  It might be burned out.  Swap in another LED from another board.  Check all the LEDs for similar dark spots.   Pay attention to polarity when swapping LEDs.

Do not test individual LEDs by connecting a battery directly to them.  Always use a series resistor.  Connecting a power source directly to an LED is a good way to destroy it.