I think you'll find that in time you will be able to make
a lather that lasts all the way through the shave. Some
factors are the amount of water you use, the water
temperature, how much soap or cream is used, and
the amount of actual lathering/agitation you do. I find
that it also makes a difference if you add water gradually
rather than all at once.

In the mean time, just reapply fresh lather from your
brush over the dry areas before you shave.

- Scott