Why can't I send some players on vacation?

[fraggert]

Currently playing an Ajax save and a couple of weeks before the winter break I asked the board for a Warm Weather Training Camp and the request is granted, couple weeks later I get an email saying my players left for winter break and immediately after another email saying players are back from holidays (because the camp got automatically scheduled on the day that Winter Break starts).

So I just individually send my players (u18 and u23 included) on a 1 week holiday, except for 7 first team players for whom the option of sending them on holiday is not available. Said players are not injured or unavailable, any ideas to why I can't send them on holiday?