Jim,
Welcome to the forums!
I will be honest with you, based on what you describing, I believe it is your technique, not the lens or the camera. Macro work takes a lot of patients and practice. Having dedicated 1:1 macro lens, that allows you to closer to the subject than your Tamron would take even more practice and skill development.
Tamron is a very capable lens, so is K10D, take a look at our members using this lens for the subjects you are after:
http://www.pentaxforums.com/forums/p...ro-option.html
Even 18-55 is more than capable producing great closups of insects, butterflies (as it can focus pretty close to the subject), just lack the focal length, so less working distance.
Here is a link to a very well respected site with tips and suggestions on macro and should aid anyone who is venturing into the "small world" of photography:
Plonsky - Macrophotography Article
Hope this helps.
Regards,
D