Nexus is a staffing agency which means that companies pay Nexus to have a certain number of bodies to help them for a given time period. This means that Nexus does not have control over weather or not you work at a specific placement, but if you continue to ask for work they will almost always have it for you. Showing up on time and simply following instructions at a decent pace will put you in the good books at most of the placements and if there is anything you should know before going to the job (hot environment, required glasses, work boots) Nexus sends you an email with all the details.
Some companies use Nexus as a recruiting partner which means that after a certain amount of time the company paying Nexus to have you there will offer you a job usually at a higher pay. I was offered two jobs through Nexus after working at different placements which were happy with my work (I only chose one though).