Quick answer: When a game won't install on some devices, the install is failing a requirement: insufficient storage space, an OS version below what the build requires, an incompatible CPU architecture, or the device falling outside the package's supported configurations. Identify which devices fail and the reason, then address it, reduce install size, lower the minimum OS where feasible, support the needed architectures, and set clear, accurate device requirements so unsupported devices are excluded cleanly rather than failing confusingly.
A game that won't install is a barrier before the player ever plays, they can't even get it onto their device. When it's only some devices, those devices fail an installation requirement the others meet. Fixing it means finding the requirement being failed and either meeting it more broadly or clearly communicating the requirements so players know what's needed.
Why Installation Fails on Some Devices
Installation checks that the device can accept the package, and fails when it can't. Common causes: insufficient storage, the device doesn't have enough free space for the install (large games hit this on lower-storage devices). OS version too low, the build requires a minimum OS version, and devices below it can't install (common on mobile, where the build targets a minimum OS). Incompatible architecture, the package doesn't include the device's CPU architecture, so it can't install there. And unsupported device/configuration, the device falls outside what the package supports (store-level compatibility filtering, or a configuration the build excludes).
So install failures on specific devices mean those devices don't meet a requirement (storage, OS, architecture) or aren't supported by the package. The reason is usually surfaced by the install error or the store's compatibility info.
How to Diagnose It
Find which devices fail and why, the install error or store compatibility info usually indicates the reason (not enough space, OS too old, incompatible). Look at the pattern: failures on low-storage devices (size), on older-OS devices (minimum OS), or on certain architectures/devices (architecture/support). Check your build's requirements (install size, minimum OS, supported architectures) against the failing devices.
Install failures happen before the game runs, so they're diagnosed via install errors and store/platform compatibility reporting rather than in-game crash capture (the game never starts). Player reports ('it won't install on my phone') plus the device characteristics point at the cause. Knowing whether it's storage, OS version, or architecture directs the fix.
How to Fix It
Address the failed requirement. For storage/size, reduce the install size (compress assets, remove unused content, use efficient formats) so it fits more devices, and consider on-demand asset delivery to lower the initial install. For OS version, lower the minimum OS version where feasible (so older-OS devices can install), or accept and clearly state the minimum. For architecture, include the needed architectures in the build so compatible devices can install. For unsupported devices, set accurate compatibility requirements so unsupported devices are cleanly excluded (and players see why) rather than attempting and failing confusingly.
Where you genuinely can't support a device (it's below a real minimum), the goal is clear communication, accurate stated requirements so players know what's needed, rather than a confusing failure. After addressing, verify the game installs on the previously-failing devices that you intend to support (or that excluded devices are clearly communicated). Reducing size and broadening compatibility within reason, plus clear requirements, is what makes the game installable across the devices your audience uses.
A game that won't install fails a requirement, storage, OS version, or architecture. Find which, then reduce size, broaden compatibility where feasible, and state clear requirements for the rest.