The bacteria involved is probably resistant to the tetracycline. Not to mention, diluting meds in water is a less than ideal way of delivery, as you can't ensure proper dosage in the proper concentration. Oral(only with certain antibiotics) or injectable are much better routes of delivery, ensuring proper dosage is received, in the correct concentration.
Your rabbits probably need to be put on a different antibiotic(s), like azithromycin or injectable pen g, possibly enrofloxacin. And even then if it's pasteurella, it may never go away completely and your rabbits may need repeated treatment throughout their life. If they do have pasteurella and these are breeding or show rabbits, this is something to be aware of as they will need to be pulled from your program, retired, and quarantined permanently due to the contagious nature of pasteurella. Only way to know what bacteria you are dealing with and what antibiotic will be effective, is to have a culture and sensitivity done like majorv mentioned.