By judicious pruning. Starting in year 3 you can begin to thin out the clump by cutting the canes at their base. Bamboo is fast. Some plants come from rhizome which is more mature than others. Some come from seedling stock.
Some species are slow to get started and then kick in.
Sometimes, the places they are planted are different. One might be getting better drainage than another, one might have been nibbled by animals etc, etc. Upon removal of the plastic you will very likely still have some viable rhizome remaining in the area. Either pull it up also, or clear-cut all the new growth for the next couple of years, or let goats or sheep do that job for you. If you do this long enough, you will eventually succeed.