I think this can definitely happen.  (That is, if I correctly understand what you mean by "losing a vocation.")

Just as anyone can lose a calling to their profession, or later feel a stronger calling to something else, I think that can happen to those in ministry.  After all, a number of our ministers are "second career" ministers, who originally started out as something else.  Who's to say their second career will be their last career?

CH asked "Is it a struggle through ministry to feel the passion and the call that they started out with?"

I think it is a struggle to maintain the passion in any profession.  My calling is teaching, and I have experienced many different takes on that profession - as a supply teacher, as a secondary science teacher and as a university instructor.  It's very easy to get burned out. The stress is enormous, especially at the secondary level, if one is assigned different courses every year. So much time gets sucked into preparation that less time and energy is available to concentrate on the students. I'm sure that those in spiritual ministry can find the parallels....  As much maligned as they are, professional development days can be a life saver, by helping you understand your profession and get you excited about the possibilities that are underway elsewhere.

It's the same in my church "calling". I'm the tech and instrumental point person (and amateur), and if anything "cutting edge" is happening in my church, it's often connected with what I do. That needs vision, which is difficult to keep up when there are simple mechanics to take care of every week - what music to choose, figuring what's going wrong with the hardware, keeping the web page updated, putting the slides together for the next service, convincing staff that perhaps the church might be less conservative in its approach to music, listening to complaints from my wife that I'm putting too much time into church. Once again, it really helps to be able to get together with others who are also on the cutting edge - such as conferences such as the "More Franchises" one last year, and coming up again next year. Keeping one candle lit in the dark somehow seems so much more daunting than when there are other candles to help light your way.

In this city we have a UCC minister who asks each year to be maintained on the roles who is a full time artist/crafts person; another who is a counsellor; a few who teach at the university; and perhaps a few others.  Some do supply work occasionally, or part time ministry.  Some are not on the availibility list.  Lost their call?  or been called to something different?

Or is it possible that they mistook the call in the fist place?  Perhaps they felt that they should become a minister - their youth leader, their family, their minister, told them that they should.  So they went though the steps, got the education, were ordained, settled, began their ministry - and then at some point realized that this wasn't for them.  It's hard to say. 

Myself, over my lifetime I sometimes felt called but something (usually finances and then poor health and family commitments) got in the way.  Even today people tell me, 'you should have been a minister', but I don't know if I really had the call, or if I was called to lay ministry.  I know that there are aspects of the job that I wouldn't be good at - like handling disagreements at board meetings and being diplimatic with the wealthy person who thinks they should run things. 

God is not always what we want God to be. We are not always what God wants us to be.

I don't think there is any single "right" call that God has ordained for any one of us. God only wants us to use what we have to serve others. Of course, there are "wrong" calls, where we neither use what we have wisely nor are of service to others.  I agree with Meredith that a "call" shouldn't be thought of as fixed in time and place. Indeed, if you have been serving a charge for a long time, your call is probably to move, so that your old charge can be rejuvenated and your new charge may share the benefit of your experience.