Most mobile problems with Candy Rush come down to browser state or connection, not the game itself. Here's what I'd check first, in order, because the quick fixes solve the majority of cases.
Game won't load or hangs on the spinner. Refresh the page once. If it sticks, clear your browser cache and reload. A stale cached version of the HTML5 client is the usual culprit, and a hard refresh forces the latest build to download. Switching from cellular to Wi-Fi (or back) also clears up loads that stall on a weak signal.
Touch feels laggy during cascades. Close background tabs and apps. The avalanche animation is GPU-heavy, and a phone juggling ten other tabs will drop frames. On older devices, lowering graphics quality in the casino settings smooths things noticeably. Restarting the browser frees up memory if it's been open for days.
No sound or audio cuts out. Check that your phone isn't on silent and that the in-game mute toggle is off. iOS in particular blocks autoplay audio until you interact with the page, so the first tap on the screen usually restores sound.
Casino login fails on mobile. Make sure cookies aren't blocked, since the session depends on them. Disable any aggressive ad-blocker or VPN that might trip the operator's fraud checks, then retry. If two-factor verification stalls, confirm your authentication method is set up in your account before you assume the site is broken.
One more thing: if the grid renders but spins won't register, your balance or active bonus may have a restriction. Check the cashier and any wagering terms before contacting support, because that resolves more "frozen game" reports than you'd expect.