strange htaccess behavior

'm trying to block hotlinking of images from just some particular domains.
I've created a .htaccess file as shown below. Some times this seems to work, and sometimes it doesn't. I'm baffled by the inconsistent results.

Here are two successive lines from the log file (wrapped for clarity).
In the first case the image is blocked, but in the second it's not.
I've also had the condition where the image is blocked for one request, but not for a subsequent one from the same site.
Notice the first entry returns a 304 status as expected, but the second yields a 200 and delivers the image even though the domain is the same for both.

Any ideas what's going on?

log file:
------- first request
66.XXXX - - [19/May/2005:10:28:49 -0400]
"GET /panoramic_horizontal/hawaii/maui/images/maui_sunset_hpan.jpg
HTTP/1.1" 304 - "http://www.xanga.com/home.aspx?user=lisalyn"
"Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.8)
Gecko/20050511 Firefox/1.0.4"
-------second request
66.XXXX - - [19/May/2005:10:29:16 -0400]
"GET /panoramic_horizontal/scenic/images/sunset_over_water2.jpg
HTTP/1.1" 200 36747 http://www.xanga.com/home.aspx?user=MILESmayLIEbetweenUS"
"Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.8)
Gecko/20050511 Firefox/1.0.4"

---- .htaccess file:

RewriteEngine On
RewriteCond %{HTTP_REFERER} ^http://(www\.)?xanga\.com/ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www\.)?myspace\.com/ [NC]
RewriteRule \.(jpe?g|gif|bmp|png)$ - [F,L]
---------
Thanks in advance for any suggestions.

..the first entry returns a 304 status as expected..
Hi,

both requests should have been served a 403 Forbidden, not a 304 Not Modified or 200 OK. So your code doesn't work at all, or have you seen 403s in your logs?
I'm not seeing any errors in your code though, except for the "L" flag, which is not needed when you use [F]. You could try your code with only the [F] flag but I don't think it will make a difference.
Are there other rewrites in your htaccess? Ones that work? Does your host support mod rewrite?

Thanks for taking the time to look at my example.

I have subsequently figured out what you surmised, that the htaccess
file is not be read at all.
I put the code into the main conf file and it works fine.
I don't know if the version of Apache is too old or if there is some directive
missing in the main conf file to activate the htaccess feature.

So right now it just a minor inconvenience when the code needs to be modified.