2 Uno; 2 com ports but IDE recognise one only

I have 2 Arduino Uno: One is on Com port 24 and the other, should be Com 20 (Window just informed me that it fund on 20)
When I upload on the first.. everything is ok When I plug the other one it refuse to change com and stays on 24. I restarted IDE with the second board but still refuse. I can see communication (from the leds flashing) with the board when I hook the USB connector. Is there a way to force to new com port?

Kind of weardo (!#??) After several "off IDE" "restart IDE" etc... it finally recognises com 20 on second board.
I changed board back to first one with com24 and the problem have changed around... I did again same several "off IDE" "restart IDE" etc... and suddently it's ok ???
Does anyone have explanation ?
Best regards to all

Here is the message that I get: It first told me to put "verbose" ON in the preference; which I have done and retried and here is the latest message (?)

System wide configuration file is "C:\Program Files (x86)\Arduino\hardware\tools\avr/etc/avrdude.conf"

Using Port : COM20
Using Programmer : arduino
Overriding Baud Rate : 115200
avrdude: ser_open(): can't open device "\.\COM20": Le fichier spécifié est introuvable.

avrdude done. Thank you.

Problem uploading to board. See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions.

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences."