What are problems that occur in networking?

normal problem occurs in networking and L2 and L1

Besides the software bugs that occurs on vendors' platforms and products (router, switches, firewalls, etc.), all networking problems come initially from bad Design and bad Sizing.

Design concerns: Number of subnets, number of VLANs, Missing routes (for example Routes to DNS, NTP or Default Gateway), mis configuration of routing protocols, Access-Lists blocking some IPs or MAC addresses (or some users) from accessing specific resources. Not having a redundant routes could also be problematic if the main line fails.

Sizing concerns: Congestion on interfaces, throughput, queue length, network loops that can create broadcast storms, DHCP (not having enough IPs to distribute or very long lease time).

In addition to the above, some physical L1 issues could be related to physical cables (cross vs straight), bad connectors (i.e. RJ45 not connected properly), physical interfaces, speed/duplex mismatch.

Hope this Help.

it's a big question really

A lot's of problem can occur in networking , take the OSI Module (7 layer)  for example.

you can find lot's of problem per layer .

even the eights layer which is individual person or users can make network  problem occur .