If in "depends" section of PKGBUILD use name of virtual package (java-runtime for example) packer could't find this package (yaourt work fine). I understand that it packer's problem, but ...
Maybe there are another way to use virtual package in depends, or way to make packer work fine.
It isn't a good idea to use non-virtual dependenses name, I think. But now I forced use this way.
Some AUR helpers will do it automatically (like yaourt) and some don't (like packer). If you use packer, you need to install a package that provides "java-runtime" yourself. yaourt is more "automatic", packer is a more "manual".