Wall Street dealmakers are the ultimate optimists. Ask them how mergers and acquisitions activity is going, and they’ll cheerily say they’ve never been busier. But now that answer comes with a crucial caveat: it’s tough getting my deals closed.

“I have a couple of gigantic deals in the pipeline but I’m struggling to make them land,” complained a top banker at one of the boutique investment banks. Another adviser lamented how five of his transactions collapsed days before they were due to be announced.

The culprit? US stock market turbulence. The fundamental drivers behind the M&A bull run of the past five years — a stable economy, companies trying to juice revenue growth and attractive financing — remain unchanged, but stock market volatility is proving an unwelcome intruder.