Problem compiling LESS to CSS

Hello

I have a problem with my web site I have no clue how to solve it.

During development "Enable Development Mode was = YES"

I was finished styling and customization, most of my code was edited in t3-assets/dev/templates.t3_blank.less.modules.less.css I added a new css styles modified old ones. After that, I disabled Development mode, Compiled LESS to CSS and upon web site refresh everything is gone!! When I turn ON Development mode, everything works fine.

I'm really not familiar with this compiling procedure, and I don't have any clue how to solve my problem. PLS help