Giant unilamellar vesicles (GUVs) adhering to supported bilayers were used as a model system to mimic ligand-receptor mediated cell-cell adhesion. We present the effect of varying the concentration of receptors (neutravidin on the bilayer) and ligands (biotin on the vesicle) on GUV adhesion and the organization of receptors in the adhesion zone. At high concentrations of both ligands and receptors, the adhesion is strong, all the available membrane is adhered and receptors are accumulated under the adhered membrane up to the geometrical limit of close packing. At low concentrations of receptors (<0.5%), and an arbitrary concentration of ligands (>= 0.1%), adhesion does not proceed to completion: the membrane is only partially bound and parts of it still fluctuate. The receptors tend to accumulate under the adhered membrane but the filling is partial. Receptors get jammed and form clusters with fractal like shapes along the rim of the adhered vesicle in such a way that the annular cluster prevents further filling of the adhesion disc. We characterize the filling in terms of a compaction factor and the final concentration. Interestingly, the closing of the ring of jammed clusters switches the interior of the adhesion disc from one thermodynamic ensemble to another. In the new ensemble the receptors sealed within the adhesion disc are mobile but their number is fixed. Under such conditions, the usually strong neutravidin/biotin bond is weak. The incomplete adhesion state can be attributed to a combination of the effects of diffusion, jamming and the competition of enthalpy and entropy on bond formation. The formation of jammed receptor clusters reported here represents a new mechanism that influences membrane adhesion.