The community spent Saturday trying to reinforce the cliffs at low tide.
Resident Carolyn McCourt said there was "no trace" of the concrete on Sunday but the group would try again.
The cottages at Cuckmere Haven, next to the Seven Sisters chalk cliffs, have become a landmark and have appeared in several films.
Work was carried out on Saturday to defend a privately funded sea wall.
Ms McCourt said the Environment Agency had stopped maintaining other land around Cuckmere and their flank was now vulnerable.
She said residents had maintained private walls around the properties since the 1940s and that they still hoped to keep the cottages standing for the next 50 years.
"They are private properties but they are perhaps the finest examples of coastguard cottages in Europe and in this setting, they're sort of magical," she added.
The Environment Agency said it had decided not to continue flood defences in the estuary because there was little flood risk.
It said it still regularly cleared shingle from the mouth of the river.
Concrete laid by volunteers to shore up a stretch of Sussex coastline and save cottages perched on a cliff edge has been washed away overnight.