I've never been over 140 and I've had constant interaction all the time. I farmed enough souls and humanity off Queelag (helping) that I was able to join sunlight at 15 faith (probably less), ascend my pyromancy flame, and feed over 20 humanity to daughter of chaos. I also helped a LOT of people with Gaping Dragon, some with Capra, and some with Gargoyles. I got summoned against Sif once, but the dude wasn't good at healing phantoms with his estus, so I got smoked.
I loathe being human because I get invaded without fail, either by vengeance seekers or dark spirits... Hell, when I was trying to kill Kirk in Lost Izalith so his armor would spawn, I got invaded as I ran into Izalith, then again before I entered the shortcut. I don't remember why (kindling, maybe?) but I was human in Anor Londo at some point and got invaded by a dark spirit and vengeance simultaneously, the vengeance killed the dark spirit somewhere else, and I had a second dark spirit show up.
I summoned help on my initial playthrough (between levels 35 and 90ish) for the following bosses: Quelaag, Priscilla, Gwyn.
Best advice:
If you want to be invaded, stay human as much as possible.
If you want to be summoned and it isn't working, use a farm route... drop you sign, farm. If you're not summoned, hit the bonfire, then redrop your sign to change servers, farm more.
If you find none of this is helping, try making sure your ports are open and such for NAT or whatever.