1: Travel Reservations is a dog eat dog link development assignment. It's up there with mortgages, loans, and casinos. It's tough. Expect a jaded and cynical response to your emails. You might be better off phoning them.
2: Construction... My first guess is that many of the related companies don't have a reciprocal links page, and if they do, they don't put much focus on internet sales. No surprise considering the relatively small search volume, according to the Overture tool.
Maybe I'm mistaken about your niche so go ahead and correct me, but it's not one that seems to be on board, nor apparently needs to be, with an online strategy.
My answer is that it's not that you're doing anything wrong per se, but that your approach needs to be modified because it's not appropriate for your situations.
For the travel niche, you'll have to formulate an aggressive and proactive link development strategy. Did you see the battle sequences in Lord of the Rings? Ok, something like that.
For the construction widgets deal, you have to get realistic about what can be done there and formulate a strategy that is appropriate for it- for instance, an aggressive directory submission campaign.
You have to realistically assess your niche and understand the realities of what other people are doing within that niche.