Occasional failure of programs to start sandboxed

[colack]

I am running Sandboxie 3.72 32 bit on W7
Occasionally, and after the computer has been running for a while, my forced sandbox programs (email and browsers) refuse to start sandboxed. I can get them to start by disabling forced programs and running them unsandboxed or by re-booting the computer, then it's OK.

Anyone any ideas please?