There are RIs that are completely resistant to Baytril. Doesn't matter whether or not you catch it early or late, the bacteria is completely resistant to the antibiotic. There have been whole papers written on the over-use and subsequent growing bacterial resistance to Baytril in both veterinary medicine and the livestock industry.
http://evolution.berkeley.edu/evolib...050915_baytril
http://www.rabbitnetwork.org/articles/growing.shtml
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC89461/
Cut to "Discussion" for the pertinent info.
Part of the reason - a combination of every hack vet blindly treating RIs without culturing and animals not getting the full treatment - the owner sees improvement and stops administering the drug. Then you couple that several animal for food industries use it prophylactically to treat large lots of animals and you have all the making of an antibiotic that isn't as effective as it once was.
When you get the culturing results back from the lab it tells you several things - first, what bacteria is responsible. It also will rank the effectiveness of several antibiotics against the particular strain of bacteria.