The control of Tenasserim coast  in the early 18th century was divided between Burma and Siam, with the Burmese controlling down to Tavoy  and the Siamese controlling the rest. Throughout history, both kingdoms had claimed the entire coast--, and control had changed hands several times. The Burmese Pagan Dynasty controlled the entire coast until 1287. Throughout 14th and 16th centuries, Siamese kingdoms  controlled much of the coast, up to just south of present-day Mawlamyaing. In the mid-16th century, the Burmese under Toungoo kings Tabinshwehti and Bayinnaung tried to regain the coast, first failing in 1548, and finally succeeding in 1564 when they conquered all of Siam for the first time. The Siamese revolted in 1584, and under their king Naresuan regained the lower coast by 1593 and the entire coast by 1594. The Burmese retook the upper coast down to Tavoy in 1615 but failed to recover the rest. This arrangement lasted until 1740 . During this period, Mergui  on the Andaman Sea was the primary port of Siam through which its trade with India and the West was conducted.
Question: what did they fail to retake the rest of?
Answer: the coast