I like to make contact with the disc when the lever has about one more inch of travel left in it. This keeps a little spring pressure on the disc. I've seen some new ones that engage the disk almost immediately and one can hardly hold in the handle up. You could also have oil on the disc. You should be able to feel when it engages.

Hey Dude long time no see lol . As Bay has suggested I agree that the drive disc could have some oil or grease from the axile drive chain assembly that has contaminated the disc causing this slipping situation. Otherwise as advised a simple cleaning and adjustment should solve your problem . I would suggest you remove the bottom inspection plate and take a good look at the friction drive wheel and disc and also chain tension and lubrication if needed. A simple cleaning of the disc and rubber drive wheel with a degreaser and clouth should be all that is needed. I also agree with a little tighter on the adjustment to prevent slippage which can destroy a rubber drive wheel very quickly .