Hi and welcome to the site!
There could be different reasons to this problem of the cleaner being stiff to stand upright. I had this problem with my DC14 when I was replacing the motor once and it was because I hadn't fitted the motor housing correctly. If you face the motor housing, the motor spindle is on the left for the belt. Around that there is a large plastic groove for the cleaner head to slide into (everytime the head is kicked down to start vacuuming or pushed upright)
Regarding loss of suction, this could be down to the motor housding not being fitted correctly because there's a filter housing next to it by the wheel which has to be sealed (by the seal on the motor retainer). If you could block the suction at the hose with your hand, do you hear suction leaking out rapidly? This may not happen immediately. If you can hear it, that's the air relief valve kicking in. If you can't, I believe the motor housing needs to be fitted in properly.
There is one more thing- regarding the small hose underneath the vacuum. It's vital for the hose to be fitted correctly on both ends (twisted onto the purple brush housing and then the grey rubbery/plastic end goes in the machine end)
When fitted correctly, check to see if the soleplate is in perfect contact with the floor with the machine reclined
Hope this helps :)