am i wasting my time?
 

I use the crossbow resistance machine to work out. i lift in sets of about 8-15 to failure, but i never get sore. i've heard that if you dont get sore then you arent lifting enough and are wasting your time. is that true?

No, that is not true. I rarely get sore after my workouts, except for maybe my leg workouts, unless I hadn't worked out at all in a week or two. Now if you've been working out for a while and aren't noticing any gains then you might want to consider changing your workout, but just because your arms, legs, etc., don't feel like they're going to fall off doesn't mean you're not benefiting from your workout.

forget about lifting in sets, lift until your muscles fatigue.

In other words, lift until you ABSOLUTELY cannot do any more. Then see if it hurts the next day. ;)

I've just started knocking the weights down and doing more sets... seems to make you work a lot harder

so now I start off with 25 kilos, do maybe 12-16 reps. then down to 20 kilos, then down to 15 and then 10.

it seems to be helping me work harder. I'll report once I've been doing it a few more weeks and have a chance to see more of a difference