Steven, I think the reason your eBooks are not selling better is that the links you have to the eBooks on your web site are not working. That and the fact that you only have ePub available. No Mobipocket for Kindle owners.
http://www.realmsofimagination.net/destinys_mission
When I go there and then click on the links for Goodreads or EZRead, neither link actually works. On Goodreads, I was able to find out how to buy the eBook. But it's not easy and most people won't bother or figure it out. So making sure the links work, having more outlets to sell your eBooks and in Mobipocket as well as ePub should help.
What some people do (myself included) when the first eBook of a series is given out for free, is to check out the rest of the series to make sure it is complete, in the format needed and not too expensive.
Destiny's Mission is $4.99 at Goodreads and $8.00 at EZRead.
Homeworld is $8.00 at EZRead and not available at Goodreads. But the direct links to EZRead you have on your site are broken. So that means that the third book is a bit more expensive then the second book.
On your site, you have the print version listed at $11.99 yet at Amazon it is selling for $15.99.
The Oort Perimeter is free in ePub and Mobipocket. But the rest are only ePub. Kindle users won't be buying the rest of the series.
Now, if this series was available easier in eBook in both ePub and Mobipocket and both at the same $4.99 price, I do think they may very well sell better. Also where these are available for sale,
The Oort Perimeter should also be there and free.