You shoot whatever combo you want. Sounds to me it's equipment failure. I assume from previous posts it's a 550. Had a similar experience a long time ago. Double check all the advice mentioned previously. My fix....
Made my own (I'm cheap) I used a bigger mechanics mirror (2' x 3") with a telescopic handle with a holder that I fashioned myself. Infinitely adjustable. I did buy the mirror (I didn't like it but worked very well once set up). You have to add a extra locking nut and adjust it with every die plate change to secure the mirror.
The Original Powder Mirror
Bought the light from Midway
Skylight LED Lighting Kit for Dillon 550
I can see everything. The key is vigilance.
There are also plans for powder checks with the 550 on Google.
Best of luck.