USPS Flat Rate Priority Box -- not showing up as an option for my items?

[cleoandbimbo]

None of the USPS flat rate Priority Mail boxes are showing up as shipping options for my items. (Not all listings; just some.) Anyone know how I can override whatever is happening?  I've tinked with weights/dimensions, but still not getting a flat rate box as an option. Thanks!

[nobody*s_perfect]

Try  the following package dimensions:

USPS Flat Rate Envelopes: 12x9x1
USPS Small Flat Rate Box: 8x5x1
USPS Med Flat Rate Box: 14x12x3
USPS Large Flat Rate Box: 12x12x6
eBay Standard Envelope: 11x6x0.25