Not every system-to-system warp path is open from the start. A number of routes -- especially ones bridging story regions like Augment space, the G5/G6 superhighways, and faction-specific corridors -- stay locked until you complete a specific mission tied to that route. If your fleet suddenly can't plot a course somewhere you'd expect it to reach, check whether the connecting path needs a mission first rather than assuming it's a warp range problem.
The Augment corridor unlocks in stages through the Uncaged mission chain: Retrofit opens Rigel-Rainsford and the Rainsford-Lydan/Raddan-Tera Lara branches; Volatile Combinations extends from Tera Lara through Rona Tan to Gealan, Kor Na Ron, Terazon X31, and Paradane; Recuperation continues from Kor Na Ron through Duriana, Vellamo, Ifrea 3, and Olassa; and Receding Light finishes the chain from Ifrea 3 to Orda 877, Ilmatar, and Yarda 74. Two extra connectors are picked up in-system rather than from the main chain: Autopilot: Duriana (found on Goqinug in the Duriana system) links Rona Tan to Duriana, and Autopilot: Yarda (found on Tessen in the Yarda-74 system) links Duriana and Quvolis to Yarda 74.
A handful of routes come from the Critical Path storyline rather than a faction chain: Dark Space (Talmir 86 to Ambit) needs Now and Then Part 9 from the TOS Holodeck section; Take Arms Against an Empire of Troubles III (picked up on Tera Lara) opens Hauge-Kalalisko as well as the wider Haakona/Bolarus/Balduk/Rek'Sgar/Weima cluster into Garadius, Fibona, Ty'Rall, and Beta Sector; and Twists and Turns opens Umxheli/Ash-tor into New Vulcan, while Forwards is Backwards opens the path from Nikola to Singularity Apex.
The Delta Quadrant corridor unlocks through a chain of Voyager Missions, each opening one hop: Voyage to Velixys (System J-25 through Jiiny and Malon Vortex to Velixys), Voyage to Illusys (Bartu'shakai to Illysus), Voyage to Asil-A (Velixys to Asil-A), and Voyage to Bartu'shakai (Asil-A back to Bartu'shakai) -- plus Commiseration, Waste Management Part 3, Entanglement Part 8, and Silver Scar Part 2 for the smaller Commiseration-Ba'Neth, Mnea-Think Tank, Ba'Neth-Hasak, and Tugaash-Nygea hops. Mirror Independent Space, separately, opens through the Mirror Missions chain: G5K1 to Mirror Romii, then on to Mirror Nausicaa.
The higher-level superhighway network unlocks per-faction, mostly through G5/G6-specific mission sets rather than story missions: Baelosian Space Superhighway (Ty'Rall/Draumen/Garadius into Farana and Fibona) and KSG Space Superhighway (Erendari to tIn jIb) both come from the G5 Superhighway Missions; Immortal Space Superhighway (Remme-Dela-Farana) and the large Origin Sector Superhighway cluster (Endriz/Ester/Orasi-18/Ty'Gokor/Brestant into van Vlerken, Rua, and each other) come from the Combat Research Tree; and the FED/ROM/KLG Superhighway Unlocks (OPS61 into the Xenris/Xor'Na'Tin/Kinatal/Blum cluster; Har-52/Bess/Solas Astra into Lorraine, Endron Beta, and Ishka'du; Illuminia/Gratia-3 into Zeta Kan and Cortázar; Memorium to Tesdach'koH) come respectively from G6 Build Missions and the G6 Research Tree. Two more specific routes -- What We Lost (Magnamia to batlh'lI juH) and ...Something Stares Back (Vacuum Epicenter to The Void) -- come from the G5 Klingon Missions and G5 Critical Path respectively.