Bicycling is an growing sport. There are more total bicyclists than, say 20 years ago.
Because the sport is expanding, and many, but not all wear helmets, it IS possible for both the number of helmets in use to increase AND the number of head injuries to increase.
Imagine that helmet use remained the same at 50% (I don't know the actual figure, just using 50% as an example), while the sport grows at 10% per year.
If any given percentage of riders experience accidents and head injuries in a given year, then it is still possible for both "the number of helmets in use" AND "the number of head injuries" to increase, because the total number of riders is rising each year.
More head injuries does NOT prove that helmets are ineffective, it just proves more people are riding.