Why can't I link any portals in ingress?

I took over an enemy portal by destroying all their resonators. Then I deployed 7 of my own resonators, upgrading 3 of them to level 2, but it won't let me upgrade anymore even though I have more level 2 resonators. However, it won't let me deploy the 8th resonator, instead there's a link button, but whenever I click it, it says no linkable portals found. There's another couple portals owned by my faction just 163m and 200m away that I have keys for, but if I click on it on the map, the link button is greyed out.

Why can't I deploy that last resonator, upgrade more resonators to level 2, or link to anything nearby.

Turned out that the inability to deploy the 8th resonator was a bug - I wish I'd taken a screenshot. When I got back to my computer, I looked and 8 had been deployed. It must have been I kept tapping deploy on my phone faster than something could handle it, and it just didn't show up. I'm wondering if restarting the ingress app on my phone would have fixed the issue as well.

As far as being unable to link it could have been for a number or reasons that I didn't know at the time. Portals link distance is determined by the level of it's resonators, and I may not have had sufficient resonators.

Level   Range
1       160m
2       2.560km
3       12.96km
4       40.96km
5       100km
6       207.36km
7       384.16km
8       655.36km

It also could have been because the link I was trying to create crossed the link between some other portals. It wasn't obvious to me that this wasn't possible at first, but makes sense and you get a message telling you why you can't link to a portal if you click confirm even though it's greyed out.

Finally, it could have been because portals only allow a certain number of links. You can only have 8 outgoing links and 8 incoming links. When this happens you'll see something like "Origin Link Capacity Reached" when you try to confirm a link to an unlinkable portal.

As explained here a single player (you ;) ) can only deploy 4 level 2 resonators on the same portal.

Resonator Level  /  Max # Per Portal
1                   8
2                   4
3                   4
4                   4
5                   2
6                   2
7                   1
8                   1

To deploy the eighth resonator you'll have 2 choices :

1. carry a level 1 resonator
2. find somebody of your faction to finish deployement (this is the better solution)

Then you will be able to link portals together (portals have to be full of resonators to be linked).