\left( \right) incompatible with something

\begin{align}
\sum_{a=1}^{n_{GL}} \sum_{i=1}^{n_{H}} \sum_{j=1}^{n_{H}}
G_{w1} G_{w2} G_{w3} |J_{a}|
( &
\frac{\partial H_{ai}}{\partial x}
\frac{\partial H_{aj}}{\partial x}
+ \frac{\partial H_{ai}}{\partial y}
\frac{\partial H_{aj}}{\partial y}
+ \frac{\partial H_{ai}}{\partial z}
\frac{\partial H_{aj}}{\partial z}
- k^{2} H_{ai} H_{aj} \\ % ie k^2 * w * p
& + 2i
(
M_{x} k H_{ai} \frac{\partial H_{aj}}{\partial x}
+ M_{y} k H_{ai} \frac{\partial H_{aj}}{\partial y}
+ M_{z} k H_{ai} \frac{\partial H_{ak}}{\partial z}
)
)
\end{align}

works. Changing the parentheses to \left( and \right) fails, reason unknown.

It's not all bad news, though. This form works:

\begin{equation}
\begin{split}
x & = y \\
& = \left( z \right)
\end{split}
\end{equation}

Watch the direction of the new line delimiters.

More fiddling showed a line break could not occur between the \left( and \right). ?!

http://www.latex-community.org/forum/viewtopic.php?f=46&t=4823

showed that terminating a line before the break with, \right.
and starting the next line with \left.

fixes these problems.