For one product in a category, more category items doesn't show anything even though there are other products in the category and the other products in the category show items in more category items. I'm trying to figure out what is going on by debugging the sql query. It ends up being the following, but then eliminates the product 4056, since that is the product being viewed.
SELECT TOP 5 CN.* FROM ac_CatalogNodes AS CN INNER JOIN ac_Products AS P
ON CN.CatalogNodeId = P.ProductId WHERE CN.CategoryId = 125
AND CN.CatalogNodeTypeId = 1 AND CN.OrderBy >-8
AND CN.OrderBy < 8
AND P.VisibilityId = 0 ORDER BY ABS (CN.OrderBy - 0)

I can't figure out your logic on
int minOrderBy = currentOrderBy - 2 * parameters.MaxItems;
int maxOrderBy = currentOrderBy + 2 * parameters.MaxItems;

In this case, the minOrderBy is -8 and maxOrderBy is 8 because the order by of this product is 0.
If I take out the minOrderBy and the maxOrderBy, it returns 5 products, whose orderby s are 25,26,27,28.

I fixed this by moving the first product in the admin, then moving it back again.