If there's a diagonal line, it means that type of transition can't be used on that edit/cut. Transitions tend to use the 'extra' frames you cut at the beginning and end of the clip on the sequence. Those 'extra' unused frames are called "handles".
So if you scrub through a clip, set the in and out points for use on the sequence, so there's unused clip at both ends. There are 'handles' available. That clip on the sequence will be able to work with most transitions.
If you simply put the entire clip on the sequence, there aren't any unused frames, there's no 'handles' for the transition to use.