Text Message Alerts Problem

As part of my action research project I am studying what effect text message alerts have on student homework completion rates. Last night I sent a text to my control group students from my World History class reminding them to answer three specific questions for class. Unfortunately, a number of students did not receive the message in the experiment group and I have yet to figure out why that was the case. I am going to have to look into the problem and solve it very quickly because my research project is focused heavily on the effect of text message alerts and the completion rate for homework projects. I have not yet collect the data or recorded the data into bars, graphs, or charts but plan to at a later date. I will again try this same procedure with the experiment group tonight and over the weekend.