Solver De-bugging w/ Basic Product Ordering Table

Hi all! Greenhorn here. Hoping for some help. : )

I'm attempting to use Solver to optimize procurement cost in ordering products with "x", "y", and "z" characteristics to meet a given demand. i.e., solving for lowest total cost (cell O30) while constraining the total order within the upper and lower bounds.

Where I seem to be getting stuck is that Solver is a) failing to find consistent solutions, and/or b) finding no solution when obvious ones do exist (when found manually).

Attaching the sheet here. Any insight would be greatly appreciated!

Thanks!