you don't say what paper (make/pattern/plain/finish/type- paste the wall/paste the paper)
if it's paste the wall you shouldn't have the problem as these are pre stretched but joins need to be 100%.
if paste the paper make sure you stick to manufactors guide lines on soak time. to long a soak will give you as many problems as to little.
if you have uneven walls you have 2 options, depending on paper finish you can try buy either a wax or chalk crayon and colour in where required.
or the harder option is to overlap by 5cm and with a snap off blade and straight edge cut a join, this wll only work on non pattern paper it is very tricky to do and requires 2 people, make sure you use fresh bit of blade each time.
some papers there is no way of fixing problem generally the more expensive ones it is sometimes better to return the paper and try a different manufactor, we've come across this time & time again with farrow & ball dark papers we now refuse to use them.
hope of some help to you, nige.
i recently had to put right a job involving a very expensive black wallpaper, the previous decorater had hung it directly over white lining paper and white lines were showing through ... the fix was to paint the wall with black emulsion before hanging ...the result one happy customer
Wallpapers can be a nightmare , I remember a job where we hanging a paper and having problems so I phoned the manufacturers who insisted their quality control department had checked this batch and they didn't have any problems , this went on for several days so in the end I took a roll and drove down there . When I got there I asked the guy to show me how he was hanging this paper without getting any problems , well it all a different story then as he explained they where only printers they get sent a design and they only check for print quality they don't actually hang any .
I think hanging wallpaper is a thankless job , at best it all goes well and the customer gets what they expect but if the papers a pig you fell like your making excuses.