When building a SD starter, it's very normal to see ups and downs in activity levels in the first week. Chances are, you will see a lot of activities in the first two days (especially in hot and humid climate) then suddenly it looks dead. Nothing to worry about, it happens to a lot of new starters. When I built mine a couple of years ago, I really thought my starter was dead by day 4. I kept feeding it and by day 6 it suddenly came to life. By day 7, I made my first loaf and that was a glorious day! My starter has been serving us very well for over two years.
To OP, I don't think the starter is ready if only a couple of days old. It will take a lot more time for it to build the strength. The bubbles built not only on the surface of the starter, but in the middle and the bottom as well. You should be able to smell it too. Just keep feeding it until it will double in volume between 4 - 6 hours. Best of luck!