Why can't I recieve Tweets on my mobile phone?

I sent the code to 40404, the number it told me to. I recieved a message back, saying that it worked and gave me some commands. After not getting any for a day or two, I checked Twitter on my computer to find out there were about 4 or 5 new tweets. I tried messing around with the settings, and I made it so that Twitter sends tweets to my mobile phone. Waited another day, and I hadn't gotten any tweets. I check Twitter on my computer again, and it turns out there were a few I've missed, again. I can send commands and recieve SMSs from 40404, but I'm not getting any tweets.

I have a Motorola W450 (Motoactv) with T-Mobile.