Shares of F5 Networks plunged in after-hours trading Wednesday, as the company reported a shortfall in revenues and couldn't provide a solid explanation why.

Revenues of $462.8 million fell short of the $467.3 million that analysts expected, according to Thomson Reuters. F5 shares were down $19.43 (15%) at $125.95 after hours.

F5's non-GAAP net income did beat expectations, coming it at $1.55 per share compared with the analysts' estimate of $1.49.

F5 was surprised late in the first quarter to see that some large deals — those in the "$1 million-plus" range — just weren't coming through, said CEO John McAdam during Wednesday's conference call with analysts. The deals were in the Americas region; the rest of the world didn't see this problem.

What happened? The company's best guess is seasonality "due to a very large number of large wins closing during a fiscal year and Q4 drive." But McAdam candidly admitted that the company hadn't pinpointed a cause.

"We obviously checked in a big way, and really — and this is sounding weird, but — really there was nothing complex that we could see," McAdam later said. "The more we looked at the pipeline going through the quarter you could see that the forecast deals weren't moving along as fast as they should."

The company did conclude that the general trend toward virtualization was not at fault. Likewise, it wasn't the result of some new competitor coming up, officials said. But analysts seemed perplexed, considering U.S. enterprise business was deemed to be pretty good in the first quarter.

Theoretically, the missing sales should appear in the second quarter — but F5's second-quarter forecast didn't knock investors' socks off, either. F5 expects revenues to be $465 million to $475 million, short of analysts' $479 million forecast.

Separately, F5 noted that it's busy in network functions virtualization (NFV), with 2,000 proofs-of-concept (PoCs) happening worldwide. Many of them involve adding security to settings such as the virtual evolved packet core (vEPC) in an LTE network.

Quotes taken from the SeekingAlpha transcript of F5's call.