Length grid changed

Up to compass 41 every lenght is according to bpm, after it everycompass is shoter, so if I copy some part it is longer in the grid!!
Any help?

time signature change? tempo change?

… and also, is the track set to Linear Timebase rather than Musical Timebase (“clock” icon or “quarter-note” icon?)

I have created a tempo track but the grid remains unequal. Track set to linear timebase. And the tempo is always the same 120bpm
But I have just noticed a change in the base. The tempo remains the same not the frames. Changed from 4/4 to 2/2
Thanks both