Hi
First, check the SAP Note 543743 - FAQs for dunning for a further research related with customizing in your system. Check also the client's master data.
If all is OK, do a research related with BTE to enhance the check of dunnable items as the last resort. Please, see SAP Note 156985 - Reference Note: BTE for dunning. I'd try with BTE 00001060 (DUNNING: Check dunnability MHND)
I hope it helps you
Regards
Eduardo