DFplayer confusion/debugging?

Hi,

I am unable to debug why dfplayer is not working for me.

I have been trying to use DFPlayer Mini (bought from two different vendors on amaon) using both the basic and more complete (1K resistor also on TX) wiring as shown in the wiki ( https://wiki.dfrobot.com/DFPlayer_Mini_SKU_DFR0299 )

I have been mainly using the "full function" code in the same URL on an Arduino Uno.

The board has been power by both the arduino itself and also through an external AC/DC adaptor at 5V.

Of the three units:

1. One unit has a blue LED that turns on and if I play with shorting I/O lines, I get it (sometimes) to play one of the mp3s on the card. Ardunio serial monitor always gives.
DFRobot DFPlayer Mini Demo
Initializing DFPlayer ... (May take 3~5 seconds)
Unable to begin:
1.Please recheck the connection!
2.Please insert the SD card!

2. Another unit has a red LED that turns on and it does not respond to shorting I/O lines. The arduino result is exactly the same.

3. Third unit does not have any LEDs that turn on. The arduino result is exactly the same.

I am lost. I can not even figure out where to look for about what to expect for the LED behavior. I think since I am able to play one mp3 file my SD card is created correctly.

Any suggestions are greatly appreciated!

Thanks.