I am new to my work and need to collaborate with another person in my?
Individuals and whole corporations may develop a silo or stovepipe attitude. Unless their own area department, division, or sector has defined a matter a priority, it goes to the bottom of the pile of work. They may get very uppity about any commitment of time requested/expected by a mere junior peer. This may be a passive-aggressive behavior. Or the nature of your request is threatening to their status or responsibilities. The solution is to cc: your respective managers about the unfortunate cancelations, and the need to reschedule under the tighter timeline. Your manager and your coworker's manager will be able to clarify the appropriate timeframe and terms of cooperation and priorities.