What's happening is that by applying transparency, you're invoking the CMYK Transparency Blend Space (found under the Edit menu). That will convert objects on the page to CMYK at printing time. This will change bright PMS colors to duller CMYK colors.
I'd suggest changing your background color to CMYK, or ditching the transparency effect. Or else apply transparency on all your pages so they'll look the same.
Also, you need to be aware that in order to print transparency and spot colors together at print time you printer has to turn on overprinting to render them correctly. Otherwise you'll see a white box behind your drop shadow effect:
Thanks Steve! This helped a lot!