INTERRUPT problems

Question

In Atmel Studio 7 debug mode: Debugging a program written in Assembler.
I have tried to make my serial 0 tx create an interrupt. (UDR0 )

• bit 7 in SREG is set.
• Serial 0 TX com interrupt vector is correct too.
• Serial setup enables interrupt on TX char.
• program sends a char to UDR0.

DOES NOT WORK.

Tried the same with ADC:

• bit 7 in SREG is set.
• Interrupt Enable is activated in ADC- control register A
• ADC Conversion complete vector is set to correct interrupt vector.
• when ADIF goes high, the debugger Screen clears (obviously something happens ), but it never returns to calling point (after RETI).

This raises the question:
Are the INTERRUPT functions in ATMEL STUDIO 7 (7.1) in some way dysfunctional, or am I using them incorrectly?? (if so: please help).