I would take him back to square one by teaching a curved tunnel again, with you on his left. Also be very aware of your body language, you should be traveling in a slightly merging path with the tunnel and making sure you turn your feet, knees, hips, shoulders towards the entrance, also be looking at the entrance and not your dog. As for a hand signal point to the tunnel and not your dog. If and when he starts doing it make sure he is committed to going into the tunnel before you stop supporting the entrance with your body language.
Start by only sligthly curving the tunnel on the entrance end and with the tunnel shorten to the max. The tunnel should be so short and with only the first two ribs turned, he should be able to see out the other end. You can also put a target on the exit side and when he does it successfully, say Yes or click and putting a reward on the target (or if its a bait bag, open the bag and reward. Then you curve the exit end by a rib or two, slowly increase the degree of curve on both ends and slowly make the tunnel longer. If he stops going in again, take him back to the point where he was successful.
I am curious when you trained the tunnel did you train it from both sides and from all angles as it was slowly curved? And did you also train both ends?
What colour is the tunnel? Dogs that are not Hoovers for the tunnel often don't like a darker coloured tunnel. I start all dogs on my orange tunnel, its much brighter inside than my green tunnel. Once the dogs are fine with the orange tunnel, we train the green one because I don't assume that the dogs will automatically do the darker tunnel, so we start at the very being.
Don't forget just because a dog can do an obstacle from one direction or one side, you can't assume the dog will do the other end or from a different direction. It becomes a whole new obstacle or sequence to the dog, therefore it has to be trained every possible way, which means from the very beginning. The goal is to have the dog always successful and to not have holes in the training, which often leads the dog not to be successful.
If he will do the chute, I would expect it isn't a sight issue and if he will jump and turn to the left, I would expect it not to be a lameness issue.