It's hard to know, but you are in the realm of experimentation.
(These questions are more or less rhetorical. They are for prompting your problem-solving.)
- Can your spare HT hear you correctly on the input frequency? It may be worth checking, in case the HT is totally hosed.
- The SWR of third-party antennas is often dependent on specific testing environments. In this case your antenna documents requiring a "tiger-tail" to get to the SWR it claims at 2m. Perhaps you just aren't a very good ground.
- That being said, do you get the same results with the default antenna? Yaesu claims only that the antenna ought to be some minimum SWR (check the manual) at 50 ohms. But the front-end and finals of HTs are notoriously picky.
- I can't see a typical 2m j-pole being much better than a $\lambda/4$ monopole unless the problem is an ungrounded monopole not happy with its (lack of) image antenna. Which you can test with a $\lambda/4$ tiger-tail. Don't spend unless you have an idea it'll work. There are countless j-pole designs out there that use scrap wire you can test with first.
- Go for a hike to a higher location, or one away from your typical testing locations.
- Modern HTs are more computer than radio these days. Make sure it is actually working!
- Is it set to transmit at its highest wattage?
- Perhaps do a full reset of the HT just in case it is in a funny state.
- Can you hear stations?