It is a new thing I am experiencing and it happens on some of my projects. No matter the projects is heavy or not but in the project on some tracks the cut tool wont cut the clip. I mean on some tracks it cuts with no problem but on some the other tracks in the same project it wont. Does anybody have the same experience ?

have you any "take lanes" on these tracks ?
the tracks could be locked.
 
Also, are you using the "Split tool" ? place the now time where you want the split and click "S" on your keyboard.
its the shortcut for Split.

I have experienced this from time to time. One thing that sometimes has gotten it to work is to bounce the clips. I don't know what it does but this sometimes helps.

One cause of this is you can't use the scissors tool on anything with a Region FX (Melodyne, Drum Replacer, VocalSync).

make sure you are looking at the "clips" view-=-)
 

It happened to me yesterday using Alt + Left click.

Thank you my friends for your help.
The track I cant use scissors tool is a simple track with no take lane and no Region FX and it is it on clip view.
 
Here is how I could fix it : I opened a new track and cut, paste the whole clips to the new track. Problem solved. So I think it is a bug and needs to fix. Also some times as Reginal Stjohn  said it will fix it if I Bounce it to clip or track.
I don't know how should I report it.
 

Hey Reza,
This happens a lot to me also.  Generally it's when I'm using the Smart Tool.  It sometimes acts goofy.  I simply go in the Edit tool button and select Split.
It always returns to normal whenever I save, then close the session and reopen it.
Cheers.

McMoore11
Hey Reza,
This happens a lot to me also.  Generally it's when I'm using the Smart Tool.  It sometimes acts goofy.  I simply go in the Edit tool button and select Split.
It always returns to normal whenever I save, then close the session and reopen it.
Cheers.

 thanks for your help. Mine still doesn't work even close and reopen the project. Any way I think it is important feature and needs to be fix.