We have had similar issues when we have had our bulls tested. This last summer I worked at a bull stud and we did a lot of semen evaluations for bull sales and such so I gained some great knowledge I would be happy to share. Often times especially on young bulls if the person moves the electro ejaculator to a higher setting to fast or starts out to high the bull will just shut down and will not give a sample. Some models are better than others as well. I think one of the worst models is the one that is most widely used by vets. I think the best thing to do is find someone who knows more of what they are doing. We have taken bull to have them tested and the vet could not get a sample so we took them to a different vet (right after leaving the first) and he was able to get the sample just by rubbing the internal reproductive organs, he did not even need to use the electro ejaculator. Some bulls just get nervous and will shut down. Often times you can let them out of the chute and run them right back in and then they will give a sample without any difficulty.