sometimes it is better to probe after a rain when the ground is softer. right now where i live it is impossable to probe in some areas because we need rain bad and the ground is hard as a rock. probe at an angle use a back and forth motion a few inches at a time. dont try to push all the way...
heat the bottle gently in warm water under 125 deg. remove it from the water and put a little CLR around the seal. if there is any gap at all ,as the bottle cools it will draw the CLR in around the stopper and loosen any mineral deposits that are between the stopper and the bottle. be patient ...
that could be a WPA PRIVY with 2 holes. i have never dug one because the people that i know who have tried them told me not to waste my time. they do produce some bottles from ww2 and after.