It is unlikely Thai immigration would deny the 30-day tourist visa extension if you comply with the requirements.
If you haven't stayed in Thailand for too long on tourist visas, fill out all the paperwork correctly, look presentable and act patient and polite with the officers, it's highly unlikely you'd be refused. In rare cases, some immigration offices might ask you for additional documents, such as proof of address or TM.30 form, but will still issue you the extension on another visit to the immigration office.
Thai immigration doing 30-day tourist visa extensions are NOT tasked with actively using their judgement to sort applicants into accept/reject groups, but simply checking the documents and performing a routine bureaucratic procedure.
Of course, they still have the authority to deny you the extension for any reason whatsoever, so if you annoy them, they're having a bad day, you're impolite or not presentable, they could do it.