Transition to 1 sleep is hard IME. It happened at 17 months for DD. Like you I tried putting her down a little later in the morning and eventually after a few weeks she started to sleep longer once a day. Sometimes she went down at 1030 sometimes 11. Eventually we pushed it out to 1130 then 12 then 1230 which seems the ideal even at 2.5 years. DD has always been an early riser so it was hard to push out her sleeps anymore.