Custom method sometimes translates, sometimes doesn't

Question

I have this function:

public static IQueryable<Article> WhereArticleIsLive(this IQueryable<Article> q)
{
    return q.Where(x =>
        x != null
        && DateTime.UtcNow >= x.PublishTime
        && x.IsPublished
        && !x.IsDeleted);
}

And it works just fine in this query:

from a in Articles.WhereArticleIsLive()
where a.Id == 5
select new { a.Title }

But it doesn't work in this only slightly more complex query:

from s in Series
from a in Articles.WhereArticleIsLive()
where s.Id == a.SeriesId
select new { s, a }

I get this error message:

NotSupportedException: LINQ to Entities does not recognize the method 'System.Linq.IQueryable1[TheFraser.Data.Articles.Article] WhereArticleIsLive(System.Linq.IQueryable1[TheFraser.Data.Articles.Article])' method, and this method cannot be translated into a store expression.

Any idea why? Is there another way to consolidate query parameters like this?

Thanks in advance,

Rei