You need to get the glue between the two surfaces. This usually involves prising the top layer up and then forcing the glue into the gap.
Generally it is far easier the more delaminated the bat is as it is easier to 'lift' the top layer.
A more drastic approach, and one that can work if there is no access under the top layer, is to use a Stanley knife to cut across the grains in the middle of the delaminated area, lift the top layers apart, apply glue and clamp back down. This is a very risky strategy, one only to be used as a last resort.
If there is some access into the cavity I like to use a needle and syringe to inject glue into the gap and then clamp down. You may find you need several goes at this as it can be difficult to treat all of the affected areas in one go.
Post some photos.