The generic 'Not Connecting' translates to many different problems, I see this most often on my iPad but it happens on my PC/Mac too:

• Wi-Fi on 5 GHz using a channel that is not supported by your device. Supports 5 GHz does not mean EVERY channel is supported. Some devices only support a very limited range of 5 GHz channels. This is seen as no Wi-Fi signal on your device.
• DNS issue already discussed, custom DNS and not using the captive portal DNS. You have a Wi-Fi signal indicator but captive portal doesn't load.
• Browser incompatibility. I see this with iPad Safari where the browser timeout trying to resolve the captive portal is longer than the captive portal allows. Force enter an ip address 1.1.1.1 or 2.2.2.2 in the browser window. Or try to go to google.com or yahoo.com. This forces the captive portal to load.
• Browser incompatibility. Try Firefox or Chrome.
• Do you use a VPN? I use ExpressVPN and the Internet Kill Switch which stops all traffic from using a public Wi-Fi unless the VPN is up also has the effect of preventing Hilton and Marriott captive portals from working. I have to disable ExpressVPN Internet Kill Switch to get the captive portal working.

I have not tested another idea, but could my browser cookie security settings be keeping me from getting the captive portal. Do some captive portals use cookies and am I blocking some types of browser cookies needed to connect to captive portal.

I solved most of my connection issues by creating a second user on my laptop with no security settings on the browser and all defaults for Wi-Fi and DNS. I first log in as that user, connect to Wi-Fi, then logout and back in as me. The captive portal now knows my Wi-Fi- MAC address and it always works for VPN and other apps. Unless it is one of those 24-hour captive portal setups where you get to re-authenticate every 24 hours.