Earlier I used to take out the cells (max 3 months old stored at -80C) quick thaw them by rubbing in between the palms and then put in 10% FBS after centrifuging it in complete media. and the cells used to grow fine. but recently the cells (even just 1-month-old cells from -80C) do not get attached and they don't even die out in flask (tried max 4 days) they just keep floating. I tried increasing FBS and also tried changing media but it does not affect their attachment.