If the piece can be pushed back down easily enough and held there with a little weight like a flower pot or similar, use clear water based caulk to inject into the gap as much as possible. Get it in as deep as possible along the length of the gap. A fine cut tip will aid in doing so. Fill the gap, and when it is pressed down the excess will push out and around the groove a little that is broken off that you don't see. After the weight is added to hold everything down, clean up the excess and let it set for 2 or more days while the caulk dries INSIDE the space. Air will not get to it easily for it to dry rapidly, so patience is needed here.
Polyseamseal(R) Is my choice for the job. It is water based, rated as an adhesive and comes in clear. It is available in small tubes so you can use the applicator if you like.
The adhesive will ooze out, you want it to. You also want it to go in as far and wide as possible too.That is why the clear caulk. Some will be left in the groove between the planks. The idea is to have a bucket or sink close by to rinse the sponge out frequently. Squeezing out as much water as possible so water does not puddle while cleaning. The caulk will go down white and dry clear. Do not let the sponge be so wet it will wash out the caulk you are trying to clean up. That is why squeezing it out as much as possible and cleaning frequently. Change the rinse water and clean again. Make sure all caulk is off the surface only. The white line between the pieces will dry clear and it won't be seen in time, a few hours. To affirm a prior comment, make sure the weight you use will hold the plank down. With the caulk everywhere before cleanup, it is the only chance to see the gap tight until the caulk is cleaned up