It sounds to me like you have two possible root causes here:
- The developers could triage but are instead escalating
- The developers should be capable of triage but are not able to because they do not have the knowledge or skill, and so escalate
In the first instance, if you want to change their behaviours you need to do several things; you need to inform them of the required behaviour, you need to measure and monitor their on-going behaviour (i.e. track their escalations) and you need to feed back whether their performance meets expectations on an on-going basis. It is also worth pointing out that the Team Lead could surely reject their escalation and demand they do more triage before bringing the issues back to him/her. Before long they would learn that if they are going to escalate then they need to triage first.
In the second instance, irrespective of whether you believe they should be able to undertake the basic level of triage, it is clear that they are not able to for whatever reason. You need to look at your hiring policies and practises to understand how this key requirement was missed and you also need to examine options for how to increase the skillsets of the developers- this could be more hires, internal and external training, walkthroughs, peer assistance etc. But you need to recognise as an organisation that your capability is not as effective as it needs to be and find ways to make it more effective.
In respect of "We don't understand why the developer is struggling with what we think are basic common issues when they're supposed to be a senior developer with experience in the tech." - Have you tried asking them why they are struggling?