I really don't think a new vacuum cleaner will make the slightest difference, but some of the suggestions might help, provided that you're employing a good wand technique.
Most c/c's are either 'scrubbers' or 'speed merchants' and few actually allow the wand and product to do the job.
If the job is difficult, increase your overlap and slow down the wand action.
In some cases. you will get the best results, from slightly lifting as you pull back, this increases the air flow and provided you have enough power, are using good products and working against the pile, you will get the best possible resuts.
robbie