Looking back through the years, most rework and "misdiagnosis" is caused by:
#1 Lack of experience and technical knowledge. Many mechanics are good at taking things apart and reassembling, but do not know how to Troubleshoot. Having knowledge, like proshadetree mentioned, is very important. Studying diagrams and schematics as well as "principals of operation". If you know how stuff works, then you can fix it. Diagnostic skills come from "knowledge and years of experience.
#2 being in a hurry. We all know customers want their stuff "NOW" and put pressure on a fellow to "hurry" and get them going. This is a receipe for failure, you have to take the time to check it out and fix it right, some techs fall apart under pressue. This doesn't mean they are "Bad", they may not be cut out for the road, a shop enviroment may be best for them. We have all been their, The shipping guy has his boss breathing down his neck to get trailers loaded, so he is just passing it on to us, you have to learn to not let it get to you. Having good customer skills is a must in these situations.
Having a dispatcher rushing you does not help either, "Hurry up, I have another call for you," or the famous, "How much do you lack?, Joe over at ABC needs you there ASAP". My response is "Tell him to grab a number and get in line or send another tech, what is the sense of rushing through and misdiagnosing or screw up and leave something loose and have to come back?
A "GOOD" manager will use the 90 day deal to evaluate a new tech. Put him in the shop for a week and see how he does, then let him ride with seasoned Techs who know their stuff and check him out, it really does not take a long time to figure out if they are going to cut it.

Roadservice is a world all of it's own, nothing against shop guys, we need them too. You can take the best shop guy in town, give him a hand full of Work order forms and put him in a van and 3/4ths of the customers will be upset. He will beg to be back in the shop. You have to be a little bit "Nuts" to be in Road Service, That must be why I love it!! I would rather "Pich crap with the Chickens then work in a shop!
If managers would properly evaluate new hires, rework would be almost non-exsistant. But if you know nothing then how can you evaluate a new hire..... "There I go again, picking on managers, My Bad!!
More in line with the thread of the post, When you interviewed for the spot, if they explained the travel time deal with you, and you agreed to it, well hum........A wise man told me once, "Don't complain about what YOU allowed or agreed too"
You might need to pull a "Hank Snow", "Im moving on , I'll soon be gone!