DoModal main application returning -1 code randomly

I have a vc++ program that calls another large exe written in MFC vc++ over and over with create process threads. sometimes the exe fails to start because the DoModal in the exe fails, but not always. out of 6 calls, 2 may not start.

is there some sort of resource limit? or something else that can identify the problem rather than a -1 return code? It seems to be mainly when I run as a service under localsystem account, not sure why - might be a red herring.

In the main application startup main window:-

 CTestDlg dlg;
 int ret = dlg.DoModal();

return code is sometimes -1.