Bosch ART30 GSD Strimmer Ongoing Problem

I own a Bosch ART30 GSD (not combi-trim) corded strimmer which I have had for some time. It works absolutely fine with the exception that recently the spool line keeps coming of the spool during use. Initially I discovered that the ‘slider’ was broken so replaced this but it didn’t help. Since then I have also replaced the spool cassette and head cap but still non of this as rectified the problem. 

What I cannot understand is that the ‘slider’ (picture attached) just slots into a recess at the base of the fitting but it doesn’t seem to do anything. On closer inspection I think there must be a part missing as I think the ‘slider’ must be held in position and push against the spool cassette to prevent the line from coming off? Maybe by a spring or something very similar? I may be mistaken, but I thought I’d post on here to see if anyone would know anything about these strimmers? Any advice would be greatly appreciated. 

Those blue trimmers are REALLY EXPENSIVE$$$$ at $30AUD for a pack of ten. I trim grass at my house and every 10-20M I go through 5 of them! Absolutely useless and disappointing 😠🛠🔧