A lot of your long haul tours are too short in my opinion. After travelling such a distance at least a 14 night stay is warranted, would be even better if it was one or two more. I am reluctant to go on an 8, 10 or 12 night tour on long haul.
I have to add on £150-ish for a connecting flight, and in some cases £60/120 for a night or two at Heathrow. I therefore do not go on your European tours as they are too short to warrant this add-on.
I agree with some of the comments above that the tours are getting very expensive for the average person thus I expect not as many people are booking as in the past. I went to Cuba about 5/6 years ago and it took me 3 years trying to get it as it had to be booked over 6 months in advance (not always possible for me to do this). Nowadays there seems to be no problem getting booked nearer the holiday.
I would not be adverse to lowering the standard of hotels to 3* to make the tour cheaper. A company in Dublin offer a 3* or 4* option, or a deluxe and standard option on hotels to certain destinations. They also offer solo friendly holidays which are very reasonably priced, they do not have the choice that Just You do but are adding more all the time.