There are several periods in the Roman Empire that saw shortages of labour workers. But the one that is most relevant is the fall of the West Roman Empire.
The first reason relates to economic troubles. Decades of battles and wars stripped the Roman Empire of its wealth. Having no supply to compensate for payment, the labour forces of the Roman Empire will eventually fall apart.
Secondly, there was a critical lack of slave supply in the empire. The slaves were conquered people by the Romans from battles and brought under the empire. There they would make use of the slaves by having them work in agricultural, mining, and personal fields. The Romans were heavily depended on their slaves to operate their labours. But once the Roman expansion came to a halt in the 2nd century, there comes no more slaves brought by the military. Soon the slave labour force dwindled and the Romans lost one of their main source of labour.
Lastly, the Roman Empire and it’s citizens was too caught up by outside invasions and political corruptions. There exists a split in the West Roman Empire where the lack of unity accounted for less unified use of labours. More money had to be spent and enlistments had to occur to get more soldiers into the military to defend from outside attackers. Political chaos also contributed to the catastrophic events that led up to the collapse of the Western Roman Empire.
This is only my understanding and interpretation, I hoped it helped tho :)