[Tilecache] Missing some tiles

[hz hanks]

Hi, All:

I've set up the tile cache under apache and it's working well. The
only problem is that some tiles will be missing under some zoom levels
(zoom level > 16) in some deterministic position.

The configuration is quite simple:
[basic]
type=WMS
url=http://labs.metacarta.com/wms/vmap0
extension=png

And the code is as following:
var basicLayer =  new OpenLayers.Layer.WMS( "basic",
"http://localhost/cgi-bin/tilecache.cgi?", {layers: 'basic', format:
'image/png' } );

Since it's working well when zoom level is less than 16, and I can
indeed get the tiles in those positions if I access the map server
directly, I don't know where is wrong. Can anyone help me? Thanks!

Best,
Hanks