No entiendo el motivo por el cual me rechazan unas cuantas fotos de objetos de cerámica.

There is no detailed reason for a rejection given to you. If you want us to analyse, post one or two (please not more) pictures with the rejection reason given, and we will take a look at them and try to figure out what is wrong.

If you are new to stock, you should consider these resources: https://helpx.adobe.com/stock/contributor/tutorials.html
Please read the contributor user manual for more information on Adobe stock contributions: https://helpx.adobe.com/stock/contributor/user-guide.html
See here for rejection reasons: https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
and especially quality and technical issues: https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html

What was the actual rejection reason - Technical Issues, IP...?

For IP, you need to include a release form.