The following is a quote from ebay
Starting this week, the display of multiple identical items from the same seller will be limited to one in Search results. An item is considered identical if it has the same title and the same listing format, and the listing chosen for display will be determined by the sort option that the buyer chooses.
I had 10 listing of the same item, finishing within a 10 hour span. Upon doing a search, I found that only one was showing up. I called ebay and they explained to me that this is their new system and when the shown item gets a bid, another would show up. I thought to myself that the more time my items are not seen the more likely that they would not be sold.
The solution: I changed the title of 9 of the items making sure to keep the most important keywords. Low and behold all 10 of my items are now showing on the first page.
So, if you have multiple listing of the same item and you want all to show up in a search, make sure they all have different titles.
eg Listing #1 New Water Bottle
Listing #2 New Water Bottle **WOW**
It also works if you differ the starting price. I tried it by listing items with a 1 cent difference in the starting price and all showed up with minutes of each other. Have fun people.