cellphone woes

on the 21 December 2014 i took my cellphone back to the store because it was giving me problems and it was still under guarantee. I handed the phone in and was told that they would respond in a week or 2 concerning the phone. After 2 weeks i received a call from the store telling me to come fetch the phone. They said that the phone i was using could not be repaired so they were going to give me a new phone. On the 14 January 2015 i went to the store to collect the phone but then it was discovered that the phone was missing the back cover. The manager said she would call the head office and request a cover or send the phone back and promised to call me when everything is OK. 2 weeks passed and i did not receive any call, i went back to the store to enquire but i was told that they would call as soon as the cover comes but they never did. I went back to the store on the 26 February 2015 and the sales lady made a phone call and told me the cover will be send and the latest will be on Wednesday which would be march 4. Today is the 12 and its almost 3 months without a phone. what i don't understand is how can a new phone be missing a cover?