The single biggest problem I had was companies simply not replying, be that e-mail, or contact form on their website. Even calling and leaving a message.
I've had worse - they quoted, we were all ready to go down there ( 3hr trip) & they decided at the last minute they didn't want the job.
I recently sent RFQs for a job to 3 small to medium UK places I've used before -
45 PCBs (250x300mm) , single-sided 1000 parts on each, 2 BOM lines, 1 week turnround.
Unbelievable difference in quotes :
"P" £87 each, £200 stencil, free setup ( £68 for 2-3week turnround)
"K" £30 each. £150 stencil, £150 setup
"F" £13 each, £175 stencil, £100 setup
For various reasons we also got a batch of five done by JLC, assembly cost was $35 setup+stencil, $30 each assembly.
(I couldn't get a JLC quote for the full qty due to insufficient parts stock)