Debugging Windows CE 6 service - windows-services

Question

Update: I've updated the text below to reflect results of my further investigations.
I want to debug my Windows CE 6 service from Visual C++ 2008. But breakpoints in several service API functions don't get hit, while should be. And that's not all. Seems like setting a breakpoint sometimes changes the code flow.
I have breakpoints set on all the service's exported functions (xxx_Init, xxx_Open, xxx_IOControl, etc.). I deploy the service DLL debug binary to a device and then I attach the debugger to the servicesd.exe process on the device.
After I run
services load MYSVC
on the device console, the module gets loaded, all the breakpoints become enabled and then the breakpoint in the xxx_Init function gets hit. I then execute and exit the function by pressing F5 in VC++. So far, so good.
But then, I run the
services start xxx:
command. Under the hood it calls the CreateFile API which results in a call to my xxx_Open function. If the CreateFile succeeds, then the DeviceIoControl API is called, which in turn calls my xxx_IOControl.
My implementation of xxx_Open just logs and returns a non-zero value, allowing the upper CreateFile API call to succeed:
DWORD APIENTRY xxx_Open(DWORD data, DWORD access, DWORD shareMode)
{
APP_LOG_INFO() << L"The service is being opened";
return 1; // Breakpoint here
}
If there is a breakpoint set on the "return 1" line, the breakpoint does not get hit, but I've got the
xxx: is not a valid service
response on the console, as if the function returned 0. If I remove the breakpoint, then everything goes fine.
The same goes if I set a breakpoint on the code in xxx_IOControl function:
DWORD xxx_IOControl(DWORD code, /* ... */)
{
DWORD error = ERROR_SUCCESS; // (1)
switch (code) // (2)
{
...
default: // (3)
...
}
Having a breakpoint on the line (1) leaves the error variable unitialized. Having a breakpoint on the line (2) makes the code to fall to the default branch on line (3), even if there exists a case branch for the code provided. In any of these cases the breakpoints are not hit.
At the same time, if I LoadLibrary my DLL (from a test executable), GetProcAddress of the xxx_Open and xxx_IOControl and call them, the debugger works as intended, all the breakpoints get hit and the code flow seem to be correct.
What am I missing? Why the debugger interfere with the services in such a weird way? Is there a way to reliably debug the service functions (I need IOControl mostly)?