car wont start?

ford escort cabriolet 1995.....had a new starter motor fitted car running fine then 3 weeks later it would not start,tryed jump starting it no joy,so had to bump start it.once we got car running battery light was staying on so i had a new alternator fitted but battery light still staying on but my battery has been tested and is fine.once you turn the car of it will not start again unless it is bumped started.any one know what could be causing the problem.thanxs