Cohesive Elements and Soft Materials

I am working on a model in which i have a soft layer of material bonded between to rigid layers of material via cohesive elements following a linear separation-traction law based on a critical fracture energy value I determined from experimental and numerical work. I have the interlayer and cohesive elements as one part with a partitioned face and connected to the two adherends via surface-to-surface ties. I have found that my model runs successfully only if the soft interlayer material is made very stiff. I have tried many ways of defining the material, from elastic to different methods of hyperelastic, and I have tried different ways of defining the elements. From building simpler models without the cohesive elements, it appears the problem is the soft confined material. Does anyone of suggestions on working with soft confined materials in Abaqus? All I found in the manuel is that soft confined materials are difficult.