Two possibilities:
1) You're just not tightening them enough. (Unlikely, no?)
2) The overall bolt length is too great.
Why are you using two spacers? In most application with a space there is one spacer between the ring and the spider.
1) Without using a machine of some sort they are not going to be getting any tighter, so that is rather unlikely.
2) Most likely
I'm using two spacers because it is a fixed gear conversion. The bolts that were originally on there held one spacer and two chainrings. I eliminated the one chainring and was required to add a spacer to still use the original bolts. With just one spacer the bolts will hit each other and therefore cannot be tightened anymore and the chainring is then loose.