You can get a reading on a voltmeter which will tell you what potential you have,but won't tell you how much current can pass.Probably you have a bad connection somewhere.Fistly try running an alternative earth to where it should go see if the horn works,if so you need to check the earth from the button through steering column to the horn and the earth to the column itself.If you need a live to get it going check the feed from the fuse ,to the fuse and to the horn itself.Best guess is it is a dodgy fuse /fusebox connection or the horn button itself.