These are the steps you should take, to ensure a lasting coat.
Sand the bench down completely and dust off.
Use a clear oil primer so the wood will not suck any water into it.
Then exterior poly or varnish several coats sanding in between.
This is just like paint, it will eventually peel, also take into consideration that if the bench is under a tree, the sap dripping from the tree will deteriorate the finish . Also, if the product you choose is old or has been exposed to temperatures under 32 degrees while in the can or while drying (3days) the finish will not last too long. And one more thing to consider, if you keep using the same stuff, maybe it is a bad mix, try a new can or a new brand.