You don't say what methodology you're using which makes it tricky to advise exactly.
Where I work if there is a blocker, it is every hand to the pump doing whatever they can to expedite the development.
Consider if there might be a wider problem with you as the lead taking on too much. Yes, people will be looking to you for technical leadership but that doesn't mean some of your more able team members can't share the workload if they're mentored.
Aside from this, is there any other non-critical work they can get ahead on? Also, is there any work they've completed that could be further polished (refactored, removing technical debt, documentation, adding tests etc).
If there really isn't anything, give them something - go thru logs, builds, documents, test plans, designs, diagrams, write agendas, arrange meetings, hold brown bag sessions, share knowledge etc. There is always something to do. If people are willingly just sat around doing nothing on the company coin that should be escalated as they're clearly not team players.