Whoosh + django-haystack missing search results

Question

I'm playing around with a relatively straightforward implementation of Whoosh 2.6 and django-haystack 2.3.1 for a search of "contact" objects. However, an example search for "mary" only returns a small subset of my many "Mary" contacts. Here are the relevant files:

search_indexes.py

from django.db.models import Q
from haystack import indexes
from apps.contact.models import Contact

class ContactIndex(indexes.SearchIndex, indexes.Indexable):
    text = indexes.CharField(document=True, use_template=True)
    full_name = indexes.CharField(model_attr='full_name', null=True)
    email = indexes.CharField(model_attr='email', null=True)
    phone = indexes.CharField(model_attr='phone', null=True)
    content_auto = indexes.NgramField(use_template=True)
    # several more indexed=False fields to avoid db hits later

    def get_model(self):
        return Contact

    def index_queryset(self, **kwargs):
        # one of full_name, email, phone has to be non-null   
        return self.get_model().objects.filter(
            Q(full_name__isnull=False) | Q(email__isnull=False) | Q(phone__isnull=False))

    def get_updated_field(self):
        return 'date_modified'

contact_text.txt and contact_content_auto.txt (both the same)

{% if object.full_name %}{{ object.full_name }}{% else %}{% endif %}
{% if object.email %}{{ object.email }}{% else %}{% endif %}
{% if object.phone %}{{ object.phone }}{% else %}{% endif %}

views.py

def search(request):
    sqs = SearchQuerySet()
    form = SearchForm(request.POST, searchqueryset=sqs, load_all=False)
    if form.is_valid():
        return form.search()

Relevant notes:

• some code has been omitted for simplicity
• the templates have if statements in order to get rid of the word "None" in the search index
• currently, all of my Contact objects have a valid full_name
• there is no pattern to which "mary" results are returned and which aren't: select count(*) from contact_contact where lower(full_name) like '%mary%'; returns 97 rows, while the search returns only 5. Similarly, for 483 "john" contacts, the search returns 19.
• using Python 2.7, Django 1.5 and Postgres 8.4