All players get the same weekly set regardless of level. If you can't complete a current set you will have the option to skip it when a new set is available. You can also ask your friends (or make some new ones on the Pocket Frogs Facebook page) to gift you the frogs you need. When you use the "Get New Set" button the newest available set is offered.
Sets are offered in reverse chronological order. Once all newer sets are completed you will have the chance to go back and complete any uncompleted sets.
You can view the current and past weekly sets at the following user created/managed site: