Sean,
This is an issue that I've helped resolve many times. Try checking out the following possibilities, as it probably has something do with at least one of them:
1. The IP address being used - Is it the same IP thats been used to post your cars since you originally started posting them? If not, have you been using the same CL account?
2. The CL account being used - If you're using the same CL account, and your IP has changed, that could be a reason. If too many IP's access the same CL Account, it can decrease its efficiency when posting. Try making sure your CL accounts are being accessed by a single IP address.
3. Templates - Are yo using the same templates when you post your cars? If you're posting the same cars over and over, using the same templates, CL could think that you're overposting the same product (especially if you're using the same CL account), because the same code (html) is being used to generate the listing. If so, try using other templates so your postings have a different look to them.
4. Repeated Data - Any repeated information, a) the dealers contact information, b) hosted images (where are your photos being sourced from, the link to them is probably the same, c) template code, d) any piece of information that can be repeated. You may think that your posting are all unique and different, but there could be something within the html that is the same. CL filters out anything that is repeated over and over because they consider it to be spam.
aside from these possible scenarios, there could be a number of other reasons why, but its hard to determine without speaking to you personally.
If you continue to have this problem, please feel free to contact me anytime, I've helped resolve this matter several times and its usually not very hard to overcome. A lot of times its the one little thing you never considered to be a possibility. Good Luck! 
