So without further information other than 'it doesn't work' I'll have to make a guess.

If there is simply no activity then suspect the oscillator, check the crystal/resonator whatever and associated components. If it runs but crashes/freezes suspect timing so check external memory and also decoupling because if this is insufficient then a new part taking more current may introduce ground bounce etc. Also check any series ferrite beads/resistors fitted for EMC. I have had several cases of improperly marked resistors that were tough to track down so while you are at it check the actual values of all the passive components. Lastly if it works mostly but goes wrong occasionaly I would suspect a mask change. I once worked on an 8051 development where the part was made as part of a so called 'mask exchange' agreement so the parts should have been identical to the Intel originals. It turned out that for one instruction only the new part set a flag differently to the Intel part.

Ian