Huh?
What level of enhancements are you combining? What is the listed chance for success? Combining lower-level enhancements with existing higher-level ones will yeild a lower chance of success. Also, combining DOs with SOs should not be possible (AFAIK).
If combining an enhancement would take it to > 3 levels above you current level it will only allow it to be replaced e.g. if you are lvl 27 and have some lvl 30 enhancements (whether 30, 29+ or 28++) slotted you cannot combine those with another and dragging another enhancement to the slot will replace it.
That is WAI
Mind of Gaia lvl 50 Defiant's first Mind/Storm 'troller.
Deadly Doc 50 Dark/Dark Corr
and lots more on Pinnacle,Union and Defiant
there is an issue when combining DOs and SOs that out of every 5 combinations one drops at least sometimes up to 5 drops for a single slot making the amount of enhs you have to buy not only triple normal but up to 4 times as much inf. Having them drop at the last + is very frustrating and unnecessary.