how to adjust dissolve tarnsitions in FCPX

The outgoing clip, on the left, has no handles. You're at the very end of the available media. You need to trim the outgoing shot. You can do that in the precision editor by sliding the shot to the right. That will create more space, allowing you to lengthen the transition.

Most likely because there is no additional media. The transition will only use the available handles. If there isn't enough you will have to ripple the clips to shorten them and create more overlap for the transition.

If you add full clips and add a transition, you get the "nag" dialog about creating a transition. The amount of transition you get is set in your Preferences > Editing:

As Tom has said, you can open up the "Precision Editor" in the storyline by double clicking on the transition. You can add handles you need in the Precision editor then resize the transition as needed:

[Sorry - this was clipped a little tightly on the end -- only allowed 2MB for the upload.]

Double click the transition. This will open the precision editor. Are there any handles available beyond the edit point? Post a screenshot.

This area here, what you don't actually see in the timeline, are commonly called handles.