That's sad to hear - we also noticed trouble getting them to make it through to frogs. We started with about 20 or so tadpoles and they seem to get to the point just after losing their tails and would die off. We thought maybe they couldn't get out of the water enough in the tank they were in so have transferred them to water feature in the garden with much better access for them to leave so once they can get mobile they can disappear on there own.

Better luck next time

Have there been positive results yet with the transfer to the water feature?  Once I see them with their front legs out, I always move them to a container with a much shallower level of water and a rock for them to get on.  Once I see them getting on the rock or climbing the walls, I know they're ready to graduate.  

For some species, such as tree frogs, they can climb out of just about anything once they hit the froglet stage of development. Earlier in the year, I saw one froglet of a particular species of toad/frog needing help getting out of a container of water.  It was made quite apparent as a sibling in the same container outside already passed away.  Was glad I was able to help the one still alive.

I figure it must be working as I have not seen anymore dead ones. mind you if they jump in to the lower pond they run the risk of been devoured by 1/2 dozen large goldfish :o