Hi @Sayem and welcome to the Meraki Community.
Here is a lengthy answer but touches on the main points regarding roaming behavior & balancing and includes a couple tips I spotted in the other answers. This is where some of my colleagues like to accuse me of getting paid by the word, but I assure you I'm here on a volunteer basis, LOL. 😁
With all that follows below, the key thing to remember is that it will never result in a perfectly even distribution of clients across all APs in a deployment. There are MANY input variables, some of which are constantly changing.
For starters, consider running the latest stable 26.6.1 firmware if you're not already. You could check with Meraki Support first to scrub your config and confirm 26.6.1 is a good choice, just in case there's a known issue specific to your environment or AP models and features you're using.
Remember that it's ALWAYS the client's (not the APs) that ultimately make the roaming decisions, and it varies wildly by device vendor, by operating system, and by device driver versions on similar devices from the same manufacturer. I've seen devices cling for dear life to an AP 50 feet away when it's right underneath another AP. It's very device-dependent.
So we cannot control the clients, but we try to get them behaving well via best practice settings. And even then, they don't always behave well, and still varies by device type, OS and driver version. And don't forget that every RF environment is going to be different, and in a given RF environment, every client is going to have a different perception of it. Different vendors and different operating systems will use different methods, often based on RSSI, which is typically proprietary and different phone/laptop/tablet manufacturers implement in different ways.
It can also depend on if and which 5GHz channels may or may not be supported by the client, there are still many clients/radios/drivers that don't support all of the 5GHz channels. This raises the definite possibility that some clients will not roam to an AP only a few feet away because they cannot "hear" it if they don't support the channel the AP is using, so they stay associated to an AP 100 feet away. That type of issue usually gets addressed during design time and planning for the least-capable but significantly-present clients.
Even if clients DO support all the same channels the AP is using, it is entirely possible that client devices can stay "sticky" to an AP (even through a ceiling or floor) even with another AP in the same room. Obviously there's going to be signal attenuation through a ceiling/floor, but there could still be enough signal strength to keep the client happily associated at a reasonable data rate.
Newer clients that support 802.11k, 11v, 11r (all of which were there since r25 firmware) can leverage neighbor reports to interact with APs to make optimal roaming decisions, and APs can assist with client balancing (11v) and advise the client the best AP to roam to based on AP load. But still, it's the client who makes the decision and initiates the roam. And that decision process is a dynamic one with constantly changing variables, such as people (attenuators) moving around.
I've observed various roaming behaviors in my own home, with a basement office AP and an AP in a 2nd floor bedroom. Some devices will consistently roam from AP1 to AP2 when I get within 15 feet of the AP, while other devices will stay associated to AP1 no matter how close I get to AP2.
Always implement standard best practices, like setting minimum bit rates (to 12Mbps for example), using automatic power reduction, band steering to 5GHz, minimizing the number of SSIDs, using an appropriate channel width, including DFS channels if appropriate, etc. For a large deployment I'd suggest an active post-deployment site survey to best understand the RF environment and review your channel plan and power levels, determine you coverage cell size and overlap, etc. and then review that against your typical client devices.
And of course you can always get with Meraki Support to take a closer look at your specific APs and clients where you might be seeing roaming issues. Many "roaming issues" can actually be normal behavior given client dependencies and so many variables. Still, Support has additional tools on the back-end to get some insight on how the APs are behaving and communicating regarding client roaming, balancing, band steering, etc.
Hope that helps!
More info
https://documentation.meraki.com/MR/Other_Topics/Client_Balancing
https://documentation.meraki.com/MR/WiFi_Basics_and_Best_Practices/802.11k_and_802.11r_Overview
https://documentation.meraki.com/MR/WiFi_Basics_and_Best_Practices/Roaming_Technologies