Answer:
La Salle´s expedition to find the mouth of the Mississippi was first attacked and had one of its ships captured by Spanish pirates.
When they reached Galveston Bay, another ship ran aground on a sandbar, which made them lose vital supplies.
Finally, La Salle made the mistake of sailing past the Mississippi by over 100 miles, landing on present-day Matagorda Bay in Texas, so Fort St. Louis ended up being built in Texas instead of Louisiana as it was intended.