Nikkei Asia:

Manufacturers of Apple Watch 7, as the device is expected to be called, began small-scale production last week but encountered critical challenges in reaching satisfactory production performance, multiple people familiar with the situation said.

And:

Three sources said the current disappointing production quality could be attributed to the complexity of design, which is significantly different from that of previous generations of the watch, and the assemblers found issues when putting together electronics modules, components and displays.

And (SPOILER RUMOR ahead):

The next Apple Watch will come with new features such as blood pressure measurement, they said, which means production involves fitting a greater number of components into a similar size body.

Interesting that an assembly issue popped up this late in the process, as if the assemblers were not prepared for how this generation of Apple Watch goes together.

Assuming this is not a major hurdle requiring back-to-the-drawing-board retooling, this seems more likely to impact the availability date, rather than the announcement date.