This sounds like your soldering iron tip is not properly wetted, so heat is not rapidly conducted to the work when you use it. Your iron and tip may be degraded, but perhaps your procedures are encouraging this degradation.
A soldering iron tip has the paradoxical requirement that it remains wetted with molten solder, but that the tip does not dissolve into that solder and leach away. To accomplish this, soldering iron tips are built with copper cores and iron clad surfaces that can be wetted with solder, but only reluctantly. If a soldering iron tip is allowed to oxidize because it is not attended to with fresh solder as needed, it will not conduct heat to the work when you want to use it.
Something as simple as leaving excess solder on the tip when you place the iron into the stand can help - people often clean the iron before they place it into the stand, and that only encourages the tip to become un-wetted. Only clean the iron when you remove it from the stand just before you use it - if you clean it before placing it into the stand after you've used it, then the thin layer of solder that remains after cleaning with a sponge will quickly disappear as the iron rests in the holder, and your tip will require re-wetting to work effectively.
If you want to make your tip work well, you can sometimes add solder to the tip before you put it into the stand, so that it will build a solder layer as it rests in the stand. After you use the iron, assess the wetting of the tip, clean it and then re-wet it so that it remains optimally wetted, with a good reservoir of solder wicked onto the tip, to not only prevent the tip from "drying out" but also to build a deep solder wetted layer onto the iron tip cladding.
One thing you mentioned early on is that you're using 700ºF as an iron temperature, and that's actually extremely high for anything but large pieces of metal with an iron that's too small. No PCB needs 700 degrees, 650 is also probably too high, and what will happen with 700 is that unless you constantly attend to your tip to make sure that it's wet with solder, and not coated with oxide, then it will become hopelessly coated with oxide, and your iron will not heat the work to allow you to make a nice joint.
It's hard to say what tip temperature you actually need for the work you're doing, but if you're having wetting problems and you're using 700ºF, please turn this down to 600ºF and see if life is not simpler. The melting point of 63/37 tin-lead solder is actually 361ºF, so 700 is completely excessive, only required if your iron's thermal mass is too small, and you're using it on work that's too large for your iron.
High temperatures like that also tend to burn up flux in the joint, and that also makes some joints work poorly, while others work well enough just by happenstance.
My recommendation for you is to practice with some very low iron temperatures, around 500-580ºF, and also to focus on making sure that the entire surface of your soldering iron tip is fully wetted all the time. If you have a well wetted iron tip and you contact it to both pieces of the work that you're soldering, then you can expect quality solder to flow after less than a second of heating, and you can make a quality joint in only a handful of seconds.
One final suggestion is to use liquid flux, which can help to prepare metal surfaces, especially older, not "brand new" components for soldering. Always use "no-clean" flux because you cannot expect to remove every trace of "must clean" flux - why risk corrosion. Flux pens are lovely things that can add a dab of flux to cranky oxidized metals, and result in gorgeous, well flowed joints. I bought a huge 1 liter jug of Kester 959, which is excessive for a hobbyist, but will serve me well for probably a decade of hand work. But, barring the investment into a lifetime supply of liquid flux, a quality no-clean flux pen will help to make cranky, older materials solder effectively.
Best of luck!