Wow. Cool. I also grow lavender! Its nice to meet other lavender growers! I find this is an interesting plant. And I do have feedback for you. And questions also.
First, many people see lavender plants being grown in some store parking lots like Walmart. Our Walmart parking lot grows really big tall lavender. And there are some types that grow tall.
However, I have something to tell you that might disappoint you; most of the seed, plants for lavender sold on the market are not the tall growing type. They are actually small types only. So if you can't get your lavender to grow very well that is probably why. And this is good because it means you aren't doing anything wrong.
If you go to look up the specs for the different lavender plant types they will mostly quote around 18 inches... (been awhile since I looked at this, so I hope I'm not misquoting it.) But that is actually very small. And short. You probably actually have a short growing lavender type.
I've also found a lot of MISQUOTING on lavender height and size also. Like I've seen some sites claim their English Lavender grows to 3 feet high. But then when you look at seed pack specs and other sites, they don't actually quote English Lavender growing beyond 2 feet tall. AND Spanish Lavender especially stays short. Spanish Lavender doesn't usually get over 18 inches tall much, but some sites misquote it growing up to 3 feet also. And you find this pattern also in other areas where some sites claim it can grow VERY BIG and tall. But then most of the seed packs and real sites quote it often being around 18 inches and less than 2 feet tall for the bushing.
So you might have this problem.
Now the other issue is that lavender really likes rocky mineral type soil and not the other normal soils you'd use. (You could also look at this as an issue? Perhaps wrong fertilizer type)
Before you get too mad at me...

I hope you aren't. I am trying to help. And I CAN tell you one type of lavender plant to look for;
Look for Lavender Grosso! Lavender grosso is the one that is supposed to grow the biggest and tallest. Its the one where you go to the Walmart parking lots its HUGE and covered in bees. (if its watered. I love bees so I'm happy with this.)
Not all lavender types will grow big. And many store sold lavender items, they just want sales and are cashing in on the fad. So they aren't going to usually have a tall variety.
I also found my lavender grew VERY SLOWLY. It wasn't fast and within a few months like they make it sound on the seed packs. Mine is barely getting tall enough to sound and look like it should be for a Lavender Grosso type. And I'm in my 2nd or 3rd year of them being in the yard; the first year in the yard they DID flower and were nice but very short the first year. I suspect many other people growing lavender are going to see them grow VERY SLOWLY the FIRST YEAR. (And that's OK, since they will keep growing after first year.) (So far it seems like lavender seems to grow a bit similar to currants in some ways? Currants take awhile to grow and thicken out their body.)
Edited in later; Lavender dentada is also supposed to be a taller variety. But when you look it up, the hidcote variety is supposed to be 1 of the shorter types.)
...
Side note; Have any of you tried to do lavender from seed? And what works best for this? (When I got my lavender grosso I couldn't find a seed pack for it so I just had to order from out of state as seedlings.) I'm also curious of any types of propagation methods or seed growing tricks for lavender specifically? Its supposedly harder to sprout lavender from seed compared to other things partly due to its nature and small seed size.
Thank you.