There can be many reasons for a site for not getting spidered.Some of the methodlogy while making a site which could have gone wrong are:
Frames:
There are many pitfalls in web designing that can hinder a spiders ability to completely (or even partially) index the content within a given page. One of the biggest mistakes webmasters are still making is that of using “frames”.
Frames are a design convention used to utilize independant scrollable regions within a single URL designation (or so it appears). Frames are actually divided into multiple URL designations and are generally not spiderable by the search engines.
Javascript
Again, real pretty but excessive use of web scripts in java is generally not necessary.The reason for this is that spiders cannot read javascript and thus any link or text portrayed by such Javascript will not be counted in your indexing and thus produce lost opportunity. So if you want to use java then donot use javascripts for mavigation use HTML text links .Your images can be called in Java as with your pretty little rollovers but when it comes to things you want indexed Java is not the way to go.
When calling javascript editors place the Java protocalls in header of your page: In order to save file size and maximize load times cut out that java protocall and paste it in an external file (say for eg call it java.js) then to call the java place just this one line of code: this reduces the filesize considerably with no adverse side effects.
Flash
To have a site with a flash image is great, it will impress your visitor and increase the estethic quality of your site for sure, but this flash must still be accompanied by a healthy portion of good old fashioned text, written in html, and readily accessible to the SE spiders.Limited use of flash should be done allthough now Google does spider flash site as well but still not as the simple HTML sites are done.