I'd go with the too cold bit as well, especially if they were looking nice and healthy before planting out. Could be they weren't hardened off enough before planting out. Perhaps a little fleece to keep it a bit warmer and not so draughty might help until it warms up a bit? I've planted some dwarf beans in a poly-tunnel and they are a bit sulky and slow growing, so can imagine they would be unhappy outside.