Why don't you get full stack traces when enabling the user mode stack trace?

I'm troubleshooting some memory fragmentation problems and I've been trying to figure out why things are being allocated and who's ultimately doing the allocation. So I enabled usermode stack traces for the process (+UST flag in gflags) and got a dump. When I analyze the dump and use the !heap -p -a Some_Address. I see a stack trace, but it's definitly not a full trace. I usually only see 4-7 functions in to the trace and then it stops. No errors are reported in the stack, but unfortunately it doesn't have enough information. I checked a bunch of the allocations and they all seem to have this same problem. I thought it might be the size of the stack database, but I would have expected to lost entire entries instead of just loosing part of them. Is there something I can do to increase the total size of the viewable stack. Some examples are below of the stacks I'm seeing.

0:000> !heap -p -a 3cb49008
    address 3cb49008 found in
    _HEAP @ 80000
      HEAP_ENTRY Size Prev Flags    UserPtr UserSize - state
        3cb49000 0fdd 0000  [07]   3cb49008    07ed0 - (busy)
        Trace: 6b69
        7c855014 ntdll!RtlAllocateHeapSlowly+0x00000041
        7c83d9aa ntdll!RtlAllocateHeap+0x00000e9f
        776bcfce ole32!CRetailMalloc_Alloc+0x00000016
        77d0404a oleaut32!APP_DATA::AllocCachedMem+0x0000004f
        77d04341 oleaut32!SysAllocStringByteLen+0x0000003c
        77d03f9b oleaut32!ErrStringCopyNoNull+0x00000016
        77d0456f oleaut32!VariantCopy+0x0000007e
        3ff1946 xxxx!_variant_t::_variant_t+0x00000016


0:000> !heap -p -a 2774cfc8
    address 2774cfc8 found in
    _HEAP @ 3cc0000
      HEAP_ENTRY Size Prev Flags    UserPtr UserSize - state
        2774cfc0 0008 0000  [17]   2774cfc8    00020 - (busy)
        Trace: 7de8
        7c855014 ntdll!RtlAllocateHeapSlowly+0x00000041
        7c83d9aa ntdll!RtlAllocateHeap+0x00000e9f
        4f6ad17 xxxx!malloc+0x0000007a


0:000> !heap -p -a 3ca25e08
    address 3ca25e08 found in
    _HEAP @ 80000
      HEAP_ENTRY Size Prev Flags    UserPtr UserSize - state
        3ca25e00 0007 0000  [07]   3ca25e08    00020 - (busy)
        Trace: 8588
        7c855014 ntdll!RtlAllocateHeapSlowly+0x00000041
        7c83d9aa ntdll!RtlAllocateHeap+0x00000e9f
        776bcfce ole32!CRetailMalloc_Alloc+0x00000016
        77d0404a oleaut32!APP_DATA::AllocCachedMem+0x0000004f
        77d04341 oleaut32!SysAllocStringByteLen+0x0000003c
        77d03f9b oleaut32!ErrStringCopyNoNull+0x00000016
        77d0456f oleaut32!VariantCopy+0x0000007e
        4f35abd xxxx!std::_Construct<_variant_t,_variant_t>+0x0000004d

On 32-bit Windows the system uses EBP chain to take a stack trace. You need to disable FPO optimization (/Oy-). On 64-bit Windows you will get a good stack trace even with optimization.