If you are working on many different sites with different domains, then you can submit 50 URLs from each one in a 24-hour period. Some allow only one per domain. There are several reasons submissions fail. The Host server is non-operational during spidering. The host server contains the robots.txt exclusion tag.
Pages containing frames require special formatting.
See HTML guidelines on the Internet for information on how to structure your tags. Pages requiring a cookie will not be indexed by most robots. Some engines must be able to resolve the DNS name. If there is a problem resolving the DNS name, use a static IP address. Engines having pages per domain per day limitations often max out.