continuing Deependra's answer, here is the link to the pRFC configuration (7.0, a bit old, refer to your version for accurate info) : http://help.sap.com/saphelp_nw70/helpdata/en/6e/3f560a3e0f6f4a90fb1e7db1388d4d/frameset.htm
also make sure how your RFC server group is defined (RZ12). How many servers do you have. Are they all up (SM51) . Etc. Compare everything between the 2 systems. Make sure how many dialog workprocesses you have (SM50); and how many are assigned to aRFC.