Hi Again, just wanted to add, escapement mechanism and carriage works very well. I checked typebar springs and they also seem OK... 
Also need to add that, some keys are not giving any problem but some are always causing problem... such as "Y"

Although not a stellar typer - most ultra-portables lack the type feel of a larger portable or standard - what you described is not indicative of the Gromina. It's been years since I used my '51 Gromina, but my lasting impression was that it has a wooden type action; regardless of personal impressions, it should work as you would expect any portable should.

Aside from regular maintenance, if I recall correctly there's a bottom cover on the machine that has an opening for a part of the machine's mechanism. Make sure that there's adequate clearance below this opening; missing or flattened feet would cause an issue with reduced clearance. If that's not the issue then you'll have to open up the typer and investigate further, but please do so in the Repairs and Maintenance sub-forum.

Under the type-key levers, there is a metal, vertical "comb" with narrow slots through which the levers travel.  I have solved slow-returnng key-tops on some of my machines by carefully opening up the required slot to make it just a tad wider.  I use a long narrow flat blade screwdriver to make the adjustment.  Go slow because if you open one slot too much, the adjacent slot will get tighter and then the problem moves to the adjacent type-key.

Thank you guys @pete E. and @Laurenz van Gaalen.
I willcheck the comb and try to adjust from there.

Hope it gets bettter