That's strange. I've received one of the 450s, two 150s and two 75s just in the last two days. I've only had one time, right at the time time they changed the game recently, that I didn't receive all of my survey credits within about 10 minutes.
Actually qualifying for a survey is usually very difficult. Once I qualify and complete the survey, though, it hasn't been a problem. To the tune of over 11,000 survey credits this year.
I wonder why so many people do have problems with them and I'm not. I guess I'll just keep plugging away and counting my blessings until it quits working for me as well.