A file photo shows a second stage being lowered into place atop the interstage of a Boeing Delta 2 rocket at the launch pad. The silver structure above is a work platform. Photo: NASA

Boeing has delayed the next flight of its Delta 2 rocket while engineers struggle with a yet-unexplained problem fitting together two stages of the vehicle on the launch pad.

The trouble began last week when technicians hoisted the second stage atop the awaiting first stage and attached interstage adapter during routine pre-launch assembly at Cape Canaveral's pad 17A.

The stages didn't unite properly, causing the workers to suspect a problem with the interstage. The interstage -- a hollow section that joins the first and second stages -- was removed and replaced.

But a second try to attach the second stage failed because of the same problem in which the stages did not seal together in a perfect circle.

"The stages physically don't fit together," Boeing spokesman Walt Rice said. "We are working to try to determine what is causing the problem and how to solve it."

It is not yet known which piece of the rocket is to blame.

See an illustration of a Delta 2 depicting the various parts of the rocket.

The extra work has postponed the planned June 15 launch date by six days to June 21. The rocket will carry a replacement satellite for the U.S. Air Force's NAVSTAR Global Positioning System navigation network.

Once the second stage is finally in place atop the rocket, the third stage and GPS 2R-5 satellite will be added along with the payload fairing nose cone.

Liftoff on June 21 will be possible during a window of 6:59 to 7:26 a.m. EDT (1059-1126 GMT).

This will be the 279th launch of a Delta rocket since 1960 and 33rd to carry a GPS satellite dating back to 1989.