Your photography skills are coming along nicely! Depth of field can be tricky. It can be affected by a lot of factors. The main ones being your aperture, the distance you are from the subject and if you are 'zoomed' in at all. Many lens won't allow for as low of an aperture if you are zoomed in. And some lenses allow you to get lower apertures than others. You have probably already read all about it though. It looks like you are getting the hang of it pretty well.
I can't wait to see how your blog evolves as you develop your camera skills. You're gonna be unstoppable!