Agents can become useful again if there is an issue with a survey etc and you need to negotiate on price.
Once, when I was buying and needing to renegotiate, going through the 2 solicitors was proving to take ages and somehow the exact message was getting lost in translation between ourselves, 2 solicitors and the seller. We explained the issue to the estate agent and they went direct to the seller - it meant just one intermediary that I'd spoken directly to. And because an Agent is dependent on the sale in order to get their commission, they are often more motivated to sort things out if there is a risk of it falling through.
The other thing agents are useful for is sorting out exchange and completion dates. I think there is some restriction on exactly who needs to agree the exchange and completion date, in terms of solicitors, but it with agents...so they are often involved again in liaising and helpimg agreement about these dates to be reached.
Agents are often useful too when sorting out surveys - so access to keys etc.
So actually, although you can find some significant time passes with no contact with agent, you'd expect to have contact at some point. Given its the seller who pays the agent in most cases (I have heard of set-ups where the buyer pays - the fee is taken off their offer) the agent is usually more interested in speaking with them, after the offer is accepted and things are under way......but at the whiff of the sale falling through, you'd expect them to become much more active again.
With agents, if I ring and they aren't available, I always say I will ring back rather than waiting for them, and the call that never comes. I ask the person I speak to whether I should wait an hour or two before ringing again. I always then make clear that I'm ringing for the second/third time. If you want contact with them, you should be able to have it...but sometimes you do have to be a bit more assertive.