What are the problems with teaching Fractional Measurement?

Many examples of Measurement tend to be very abstract. For example:

Instead of a ruler, think of a number line. The distance between 0 and 1 can be divided into equal lengths. For example, the distance can be divided into 5 parts to create a unit of measure called one-fifth. Marking a point on the number line and labelling it as 15 means the point is a distance of one unit from 0. The unit 15 can be divided into smaller parts. For example: dividing 15 into two equal parts creates the new unit of 110. (http://topdrawer.aamt.edu.au/)

Sometimes Teachers use very confusing Language...

They have troubles doing problems like these: 16+1816+18 212?134212?134 I've explained them to go 1) find the LCD first and then 2) put the fractions in an equivalent form (i.e. 1/3 = 3/9) and then 3) add or subtract. At first I thought that by breaking it down to these steps the kids would understand but they get confused and forget. For example 212212 could be rewritten as 224224. I then tell them to do the following operation: 224?134224?134. I tell them that they're really figuring out this: (2 - 1) + (2424 - 3434). They get really confused. Then I tell them to look at the fractions for now and ignore the whole number. They still get confused. I am up to my wits end. Not only that but they have trouble with LCM. All they want is to leave the room and not look at fractions. How can I get through these kids?! (Teacher inquiry, http://math.stackexchange.com/)