[Antennas] TUNER_WONT_TUNE_ANTENNA

[George K]

My question is; how can I make my 270 foot flat-top fed with 200 feet of 450 ohm ladder line and terminating at a 4:1 balun outside the shack where it takes 15 feet, approximately, of 50 ohm coax into the house and to my auto tuners, tune the low ends of 160 and 80?
I like this antenna as it is basically a all frequency non resonate antenna. I'm fighting putting up dipoles!
I find that my rigs internal tuner baulks at anything below 1.875MHZ on 160M and 3.620MHZ on 80M. All other bands and frequencies up to 30 meters tunes fine. I do a lot of CW DX'ing.
My question is; If I lengthen the flat top from 270 feet to, say, 290 feet will this help my lower ends of 160 and 80? It is extremely inconvenient to alter either the ladder line or coax lengths. The flat top is the easiest to alter as both ends are on pulleys and I can lower them in minutes.
Anyone got suggestions?
George