pre_get_posts filter not running

Hi, I’m working on a site running version 3.5.1 of WordPress with the WP E-commerce plugin installed. The client wanted to exclude a number of posts from the search results page, so I installed the Search Exclude plugin (http://ww.wp.xz.cn/plugins/search-exclude/). I gave the plugin a test, but it didn’t seem to do anything.

After a bit of digging, I found that when WP E-commerce was active, it was stopping the Search Exclude plugin from working.

I dumped the functions hooked to the pre_get_posts filter:

Array
(
    [10] => Array
        (
            [0000000021703a1a0000000023da960esearchFilter] => Array
                (
                    [function] => Array
                        (
                            [0] => SearchExclude Object
                                (
                                )

                            [1] => searchFilter
                        )

                    [accepted_args] => 1
                )

            [0000000021703b180000000023da960eget_posts] => Array
                (
                    [function] => Array
                        (
                            [0] => WPSC_Hide_subcatsprods_in_cat Object
                                (
                                    [q] =>
                                )

                            [1] => get_posts
                        )

                    [accepted_args] => 1
                )

        )

    [1] => Array
        (
            [_wpsc_pre_get_posts_reset_taxonomy_globals] => Array
                (
                    [function] => _wpsc_pre_get_posts_reset_taxonomy_globals
                    [accepted_args] => 1
                )

        )

    [8] => Array
        (
            [wpsc_split_the_query] => Array
                (
                    [function] => wpsc_split_the_query
                    [accepted_args] => 1
                )

        )

)

_wpsc_pre_get_posts_reset_taxonomy_globals and wpsc_split_the_query appear to run fine, however, the last two aren’t running at all.

After a bit of playing, I found that at the end of the wpsc_split_the_query method, there is the following:

remove_filter( 'pre_get_posts', 'wpsc_split_the_query', 8 );

If I comment that line out, all of the hooked functions run. Leave it in, and only 2/4 run. I tried outputting the list of hooked functions again, after the do_action, and the two functions that didn’t run were still hooked, so they hadn’t been removed, but just didn’t run :/

Does anyone know why that would be the case?