Why can't my hosts communicate with each other?

Question

I've written the following Mininet topology for k-ary fat-trees.

from mininet.topo import Topo

class FatTreeTopo(Topo):
    def build(self, k=4):
        # k is the parameter that determines the size of the Fat-Tree

        # spine switches
        spine_switches = [self.addSwitch(f"spn{i}") for i in range((k//2)**2)]

        # |pods| = k

        # fabric switches
        fab_switches = [self.addSwitch(f"fab{i}") for i in range(k**2//2)]

        # tor switches
        tor_switches = [self.addSwitch(f"tor{i}") for i in range(k**2//2)]

        # servers
        servers = [self.addHost(f"srv{i}") for i in range(k**3//4)]

        # Connect core switches to aggregation switches
        for c in range((k//2)**2):
            for pod in range(k):
                f = pod * (k//2) + c//(k//2)
                self.addLink(spine_switches[c], fab_switches[f])

        # Connect fabric switches to tor switches
        for f in range((k**2)//2):
            si = f//(k//2) * (k//2)
            for t in range(si, si + k//2):
                self.addLink(fab_switches[f], tor_switches[t])

        # Connect tor switches to servers
        for t in range((k**2)//2):
            si = t * (k//2)
            for s in range(si, si + k//2):
                self.addLink(tor_switches[t], servers[s])

When I run this code for k=4 none of my hosts (named srv#) can ping each other (tested by running pingall). For example srv0 ping srv1 returns the 'Destination Host Unreachable' message, however both the hosts are connected to the tor0 switch and can ping the switch, so I'm unsure why the switch does not forward messages between them.

The same code does work for k=2 and k=3, and I can successfully send messages between hosts.