To view All the Shifts within an Opportunity, you must adjust the date-range drop-down list on the Shifts page. By default, the Schedule shows Today & Upcoming Six Months, which shows the shifts happening within the next six months (within about 182 days from today). You can switch to other ranges including Beyond Six Months for shifts after that, Past Dates, Custom Date Range, or All Dates. Each option shows the number of shifts in that range next to the option name.
Any shift created in the opportunity appears when showing All Dates. When showing All Dates, depending on the opportunity length and the number of shifts, you might see a slower load time of the schedule views. If your opportunity has a large number of shifts, using the Custom Date Range option, as opposed to All Dates, might make it easier to find the shifts you are looking for and need to access.