Build on x64 platform won't run on 32 bit

I've got a 32bit package built on InstallShield running on Win 7 x64. When I install it on another x64 platform it installs and runs OK. If I install it on a 32bit platform it fails to run and the symptoms seem to point to WinSxS. I have checked in Prerequisites and only x86 merge modules (e.g. Visual C++ 8.0 CRT (x86) WinSXS MSM) are included.

Any ideas where else I should look for a possible cause please?