Welcome to Just Answer. I'm sorry to hear that your little isn't responding to the therapy at this point. Honestly, if it isn't getting better there is most likely something else going on in the cervical / neck area to cause the discomfort - more specifically intervertebral disk disease, or other compression on the spinal cord. Pressure on the spinal cord can cause discomfort like your describing. What is causing the compression is up in the air at this point. To get further information, films of the neck and other advanced imaging would be needed. Based on what is found, then more therapy can be presented. At this point, there isn't anything more medically to add in to the current meds besides strict rest / cage confinement for a couple weeks. If it is that painful, surgery starts to become the fore-runner for treatment. Here is a good link to read.
The x-ray your vet took is good at evaluating boney changes. They aren't the best at evaluating if there is a compression on the spinal cord or dynamic instability. These require MRI or a mylelogram to fully evaluate. If this is a muscle pull, it should be something that has resolved in a week or so. I wouldn't expect a muscle strain to be uncomfortable over 2 weeks.