What vendor?
depending on the vendor there are some counters that you can check to see where is the problem
If you are not making IRAT at all you probably need to trace at core or RNC level and not at Uu
other things to verify
1-data mismach at rnc level of external gsm cells (bcch, lac, ect)
2-data mismach at 3g msc level of outer gsm cell (if 2g and 3g are in diferents mscs)
3-xudt activated in both MSCs (if 2g and 3g are in diferents mscs)
4-in some vendors multi rab connections are not supported for irat