Question

When the government first shut down, what were lawmakers having trouble reaching an agreement about?

Answer 1

Their argument was on the budget.
The main cause that become a hindrance for a  law to be passed is the budget.
The congress often not agree on which budget allocation they need to prioritize when considering a bill. For example, Congress from Republican party tend to favor to allocate the budget for military while the one from Democratic party tend want to spend it on government programs.