It helps to roller the new patch down after applying it to help it seal any possible leak paths particularly when patching over parting lines or near an adjacent patch. There are tools for this that are a narrow wheel with a handle that you can roll over the patch repeatedly but you can use anything handy that will apply a localized force on the patch.

Amazon.com: Wooden Handle Repair Patch Tyre 1 1/2" Dia Roller Tool: Automotive