Why are my itouch chargers doing this?

Soo I have three itouch charger. One that came with the itouch, and two wall chargers. Whenever I charge them in the wall chargers, when they reach 75% it always says that it's either finished charging or it goes back and forth from the charging symbol and the charged symbol. Why the **** wot it just charge?? Am I doin something wrong?? It won't charge more than 75% no matter how long I keep it in there!!