For the amount of publicity under water search being a highly sophisticated, some of technology and techniques are old and crude.
If I recall correctly, ship with pinger locator actually drops a test ULB, so if multiple vessels does this, they will be chasing each others pings than the actual one. If they have too many pings in very close frequency it makes all the more difficult.
It is also a myth that navies have superior underwater technology. Oil companies and research organizations have latest technology.
30 day life period is based on the assumption ULB has been stored and maintained properly.