Look for changes if you "feeling that something is wrong" but aren't sure what.

A lot of times we have a feeling that something is wrong (with ourselves or with the group), but we cannot pinpoint what it is. 

The first step is ofcourse to recognize that feeling and make sure you dont ignore it. I ve found that my instincts are generally right on these things.. and very often things have blwon up because I didnt listen to these feelings.

Now I notice it more, and dont ignore, but if I cannot pinpoint what exactly is causing it, what do you do?

I dont have the answer yet, but I have a couple of learnings from a recent experience: 

Look at recent (~ 1 week) significant changes without prejudice. And see if that is the source of the problem. Even if not the source of the problem, it usually plays a part in improving / worsening the problem and thats an important clue. And even if you feel this change has been assimilated, it doesnt hurt to double check. 

And there might be changes you dont know about - so probe to find out what the changes are, instead of what "negative events" happened.  It is easier to get all the facts in first before categorizing them.

And sometimes, the other person involved has a "feeling that something is wrong" too but is in the same position as you and wont be able to separate positives from negatives clearly. 

And finally, if it involves another person, I have found that it helps to take extra effort in being direct, explicit, and avoid sarcasm (basically any indirect means of communication) till the matter comes out in the open.