It may be that it is already well fed. My experience is that plecos will generally only switch food types if the other source dries up for a while. Try withholding the algea rocks for three or four days and see if it goes for the tabs. Additionally, mine loves to have its diet supplemented with Broccoli florettes (came in handy when my labrador retriever also gained a taste for the algae tabs). He'll actually sit on top of it and protect it from my two Pearl Gouramis that also have developed a taste for frozen Broccoli.
