Failure to match

Many spam messages include a URL of the form

mailto:software(_at_)countdown911999(_dot_)com?subject=remove

I'm trying to use this as one of the triggers to divert mail to a spam
folder.  My recipe is:

  :0B:
  * subject\=remove
  IN.possible-spam

(I didn't have the '\' at first, but I tried adding that as a long
shot when it didn't work.)

The problem is that occasionally this recipe will match, but usually
it doesn't.  The example above matched in the log:

procmail: Match on "subject\=remove"

But the following examples:

REMOVE AT mailto:kellyw23(_at_)yahoo(_dot_)com?subject=remove
* email  mailto:mikethpork(_at_)yahoo(_dot_)com?subject=remove  *
Please Remove at:  mailto:jsmith1234(_at_)deskmail(_dot_)com?subject=remove

all logged as:

procmail: No match on "subject\=remove"

So I know the failure to match is not caused by some other recipe
precluding this one from being tried.  How can I figure out why it
didn't match?
                                                        -- Steve