I suppose it all depends on your companies attitude - however I doubt that there is an issue with the way the content is delivered.
I know a lot of people who no matter what you did, even if you stuck the damn rehearsal report to their forehead, they would not read it. That is "the thing". Calling someone out on "action items" is usually a reasonable way to get your point across. If they fail to get the hint, try a comment like "I spend my time creating these reports for a reason, to help keep you in the loop and tell you of any developments that occurred which may involve you. I do not make them just because I like sending emails. If you have trouble opening the reports, let me know and we can arrange an alternate method, but they ARE essential reading" (you may want to rephrase so that it is not so condescending, and hopefully to lay the guilt on a bit) and if all else fails, stop sending them to them, and then call them up after every rehearsal and read it out to them. Then call every second day to "check on the progress of XYZ". Two things will happen - they will get annoyed with the fact that you are keeping close tabs on them (to which you can reply with "I apologise, but these items need to be dealt with. I would normally have just sent you a copy of the rehearsal report, but time has shown that you do not read them") and two - the items that you needed done, will get done.
I know many people will probably dislike my method - however I am loyal to the show above almost everything (except safety of my cast and crew), and my attitude is that I will do what it takes to make sure that the show runs smoothly for everyone involved, and if it means stepping on a few toes to do so, then so be it. (My current motto is a stage managers job is not to be loved by all, but rather to be the one people know they can turn to when an issue arises and have it dealt with swiftly and efficiently. Being loved by all is a bonus.). Damaged egos can be repaired, however failure to communicated can result in wasted time, wasted money and injury. None of which can be replaced.
Moderation Note:
Depending on how this topic evolves, I may move this from "Tools of the Trade" to one of the more people oriented boards, like Stage Management: Plays and Musicals, or split the topic into two - the more "tool-ish" related part and the "getting the great unwashed to read rehearsal reports" part. It is a tricky one. I have not decided yet. I will sit on it for a while and see what happens, but forewarned is forearmed.