Could be several reasons. When did you see one? If it has been recently, the hummers are now in migration and you may see one then not again. Another reason is that flowers are starting to bloom and they are feeding from flowers more often. A third one is that it may not have liked the nectar. The feeder is fine but it is better to make your own. Heat the water. It doesn't have to boil (per a bird bander that has visited my yard) but it has to be warm enough to dissolve the sugar. The ratio is 1 part sugar to 4 parts water. Let it cool and then put it in the feeder. It doesn't need food coloring and there is debate about whether food coloring is harmful or not. Change the water every 3-5 days depending on the temperatures outside.

Hummers feed about every 15 -20 minutes but if it has a path if follows, you may not see it at your feeder every time. Once they use the flowers it is also harder to know when they are in your yard. They also eat bugs for protein and you sometimes see them "bugging" around the trees.

Yes they are in spring migration as noted above. The males arrive first scouting out for good place to protect for potention breeding purposes. They might stop at you place or they might move on till they find a good breeding location. Do not feel bad if this one leaves there will be more, keep a lookout.