Congradulations on wanting to do therapy work with your rottie!
My rotties failed the first time they were tested by a TDI evaluator - she owned boxers and very plainly told me she didn't like rotties - didn't think rotties were friendly enough.
After that I made the aquaintence of a Delta Society evaluator who mentored me through the certification process. She also did not pass my dogs the first time - Sierra had issues with crutches, and Jazz completely blew his recall. However, they both passed with flying colors the next time.
You can find the names of Delta Society and TDI evaluators in your area by checking their websites:
www.deltasociety.org for Delta Society (look under Pet Partners)
www.tdi-dog.org for Therapy Dogs International
Also please check out the following threads under "Working Rotties":
"TDI or Delta" and "Therapy Dog Options"
These discussions contain much information which may be useful to you. Also the two websites mentioned above have lots of helpful information.
Please post or PM me with any additional questions you may have and I'll try to help. Good Luck !!:)