My friend ignores my texts?

Ok so I'm in university and met this great friend and have several classes together. We exchanged phone numbers since the first meeting. At first things seem alright. I text him for the first time asking to meet up so we can do our course work together and he would respond. Even when I ask non-important questions he responds. But now he's no longer consistent. I would send several things and he doesn't reply. Yesterday I texted him asking if he wants to meet up se we can study for midterms. He never responded even though this was an important question that I needed an answer to. And I know he's always on his phone.

What's going on? Why is he acting like this? I feel to just call him up tomorrow and ask....