Most men find their abs don't become visible until you hit the 10-12% body fat range, so you've got quite a way to go with reducing your fat mass before you could expect to see them. Belly fat is the last fat store to be used up in most men, due to the way we store fat: it's a bit like a 'first in, last out' principle, where we tend to gain weight around our middles in the first place, and lose it from there at the end.
If you've lost noticeable amounts of lean tissue from your limbs, that would suggest that you are dieting too severely in terms of energy restriction, or that you're not eating a good diet during your weight-loss, and maybe not enough good-quality protein. Doing the numbers, if you were 80kg to begin with, then 16.8kg of that is fat mass (80 x 21% fat). If you've lost 6kg and are now 19% fat, that means that you're currently 74kg, and ≈14.1kg fat. Thus, you have lost 6kg, of which only 2.7kg is fat - this is only 45% fat lost of total weight, which is quite low. You'd normally aim for about 70% of the weight lost to be fat mass.
These numbers tell me that you need to increase your energy consumption a bit, eat a good, balanced diet which is still energy-restricted, but includes plenty of good-quality lean protein, in order to improve the ratio between fat mass loss and lean tissue loss.