For there to be a material every 5 levels there would need to be 200 materials for each type of resource, this means 200 ores, 200 ingots etc …. coming up with that many names was impossible for us (Tekka and myself) the best we could do was 180 or so for each type.
This means that there are some gaps in the 5 level rule, so there may be a 10 level gap to the next material instead of a 5 level gap.
To get around the gap you can continue to work on gathering the highest material you can that will still give skill gains.
Alternatively you can try to get bonus items that push your gathering skill up to a point where you can get the next material.