Hot Water Running Cold, But For Only 2 Taps?

[C3P0]

Hello, I have an Ariston DIA 24 MFFI CE boiler and it works perfectly, for everything other then my problem. The thing is all of the hot water taps get hot and stay like that, but the shower hot and bath hot run cold after a few seconds, the other taps in the bathroom are fine, just wondering what the problem could be?

Also, if I run the sink tap whoch runs fine next to it the bath tap would run warm, not hot, warm only.

Thanks, Callum.