Can't hit the ISS Repeater!

Hobojonesy KJ7UJX

I have been trying satellite Dxing work shooting for the ISS.  My equipment: Yaesu FT 991A, with a Comet GP-15 multi-band antenna.  I can hear people, mostly clearly, but cannot--absolutely--cannot make a contact.  In fact, I've placed a HT within earshot tuned to 437.800 and I don't even break squelch when I tx on 145.990 from the base, even when my tx/rx should be best.  Doesn't this mean I'm not reaching the ISS repeater?  I do live in a mountainous area so I know my abilities are limited, but as I said, there is time when I can hear everyone from Alaska to Mexico!  What else can I check?  I have tx Tone "on" and set to 67.0.  Any advice?  Should I now pull the trigger on a Yagi?  Of course, then, I guess I'll need a rotor...hmmm...I wonder if the wife will stand outside waving that Yagi around?

73 and thanks for any help!