Yes total agreement with Peter ,I restretch carpets all the time, and if it was in need of restretching it would probably not bubble in one place ,you could stretch and stretch and it would not necessarily come out because this particular area has been stretched by something ie a fault or spill or like others have said something put on it .By lifting the carpet and looking at sub floor and underlay etc you would get a better idea as it could be a number of things ,but a can of spray glue can be a life saver sometimes in these situations .Best of luck