You need the gunk off the established filter media to properly kick-start a cycle if you don't want to remove any of the actual media.
Rocks and decorations hardly contain any beneficial bacteria at all as the filter is essentially the source of oxygenation in your tank and where the lion's share of bacteria colonises.
Can I ask what your pH is? Sometimes cycling can cause your pH to crash, which in turn stalls the processing of ammonia. I know, I've been there, done that and own the tee-shirt.
If that's not the case, it sounds like it might be a problem with how much ammonia you have been adding in. What levels did you allow your ammonia to initially read as? Have you been continually adding new ammonia, or just waiting for it to be processed by the developing bacteria? I believe ideally, you should be aiming for your tank to process 4ppm of ammonia in 24 hours. However, don't quote me on that.