Do not take your obligation lightly once you accept an offer. Stick to the terms of your agreement. Word gets around and backing out of an agreement may endanger your chances elsewhere. Attempts to be released from agreements after April 15 will most likely endanger your status at other schools. Do not be discouraged if you haven't had an offer by April 15.
Openings occur all the time and new opportunities open as government grants are announced.
Sometimes offers will be made as late as September 1st.