Idiom: A person who is unfolding truth, changing statements during the conversation

I was talking to my team member on an issue. He was putting in new statements every time I countered his previous statement.

I think I can't use "truth" for these statements because when I countered, then the previous and current statements were contradictory. "After-thoughts" would be right, maybe.

Our conversation looks like you keep peeling the onion to reach a core of onion (i.e., truth).

Example:

Me: Why you didn't do Task1

TM: I wasn't at my desk

Me: But you sent one email to 'X' where in I'm also in CC

TM: Because I need some more info on Task 1

Me: Then why are you telling me now?

TM: I got stuck in Fun-Friday activities in office

Is there an idiom/phrase for this situation?

You could say that your team member is dancing around the issue:

Dance around (the issue)

• To improvise, tergiversate, etc, in order to avoid a question or issue.

Larson dances around the real issue of gun control.

(Dictionary.com)

To dance around the issue would be your team member coming up with different responses, that may or may not fit the question, just to avoid telling the truth.

TM was giving you the runaround ... "give someone the runaround". The Dictionary of American Slang. 13 Feb. 2017.

It's especially fitting for situations where someone is supposed to provide some service and they're trying to get out of doing so, by giving you the runaround.

Someone doing that is beating around the bush.

TFD(idioms):

beat around the bush (and beat about the bush)
Fig. to avoid answering a question; to stall; to waste time.
Stop beating around the bush and answer my question.

McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.

To speak evasively or misleadingly, or to stall or waste time.
To flush pheasants and other birds so they could be shot, British gamekeepers hired beaters who would swing sticks at likely places where the birds might be lurking. Not to go directly to such foliage but to work around it instead gave the impression of wasting time or not trying very hard to raise the birds; hence, beating around the bush.

Endangered Phrases by Steven D. Price Copyright © 2011 by Steven D. Price