Aspen only indexes lists when it contains 3 or more items,

[SteveEOLI]

/usr/local/aspen-discovery/code/user_list_indexer/src/com/turning_leaf_technologies/reindexer/UserListIndexer.java
if (userListSolr.getNumTitles() >= 3) {
// Index in the solr catalog

Perhaps that should be a configurable number, or nothing at all and just index all lists.