Players needing rested

I'm only in November and for a while now certain players keep getting the orange rest icon next to their name and I'm being advised to send them on holiday for 2 weeks... every few weeks! Why is this happening? I highly doubt it's my training schedule since I've had the same one for seasons now and they're all happy with it in their personal screen. Never, ever experienced this problem before but it's a massive pain. Help, please!!!

I had a few 18 and 19 year old's who were ready for first team, Stamina were 15+ for all 3, it took them a full season for them to get accustomed to first team football, the senior training. I had to rotate alot with a depleted squad as i sold the last years first teamers as these youth's had been used as subs for most of the last season and their attributes were progressing well.

I think maybe some hidden attribute or natural fitness is also a factor.

Not sure if your using the standard training schedules, im using modified ones, which may have contributed to my scenario.