The second half of the rosetta is the more difficult part to execute. The fishtailing technique is required to form the reversing zigzag of petals that form the slender top of the rosetta design. Most of the challenge with fusing the bottom with the top half of your rosetta occurs in the process of switching seamlessly from shaking to fishtailing. There are many ways to develop this technique.
Usually, trainee baristas pour good bases to their rosettas but find that their latte art design ends up missing many petals. Another tendency is for a barista to end up with a significant imbalance between the number of petals in the base compared with the top. Often, baristas will pour more petals in the base and then rush the fishtailing section of the design.