Bonding of Soft and Rigid Materials

I am workingon a model in which i have a soft layer of material bonded between to rigidlayers of material via cohesive elements following a linear separation-tractionlaw based on a critical fracture energy value I determined from experimentaland numerical work. I have the interlayer and cohesive elements as one partwith a partitioned face and connected to the two adherends viasurface-to-surface ties. I have found that my model runs successfully only ifthe soft interlayer material is made very stiff. I have tried many ways ofdefining the material, from elastic to different methods of hyperelastic, and Ihave tried different ways of defining the elements. From building simplermodels without the cohesive elements, it appears the problem is the softconfined material. Does anyone of suggestions on working with soft confinedmaterials in Abaqus? All I found in the manuel is that soft confined materialsare difficult.