Make sure that you cut the straw on an angle. You may have the straw cut straight and it may be clogging on the bottom of the water flask if it is flush to it.
If that does not work and you still have any issue, contact the company. They are great to work with.
You need get a good deal on the top and then trim the other end at a slight angle to the length of the bottom. The straws come extra long so you should trim to the bottle size you have. It should just touch the bottom and if you cut at an angle there should still be good flow.