CoD 4 enemy intel

Well I've collected 20 enemy intels so far and I have not acquired the achievement for getting 15 of them. Why is this so?

I'mplanningtotryandget30intelsandseeifIgetboth.I'mnotitwillworkthough,cananyonehelp?

Andifitdoesn'tworkwhatshouldIdotogetthisachievementbecauseIhavecompletedwhatithasaskedmetodo.

Thanks.