When a player experiences issues with claiming an equipment blueprint there are normally two different possibilities that cause it.
First, equipment blueprints must be claimed in order. As an example, if you have not claimed the MKII of an equipment blueprint you will not be able to claim the MKIII.
The second potential issue that players experience is that while they have claimed the blueprints in order they have not equipped them as they were claimed.
Below are representations of both possibilities.
In this image, you can see that the GAU19/JF Rotary Cannon MKI has already been added to the unit.
In this image you can see that while the MKI has been claimed and applied to the unit, and there is enough war credits for the MKIII it cannot be claimed.
The MKII can be claimed due to the fact that the MKI has already been claimed and equipped to the unit.
The next issue that players experience is that even though they have claimed the blueprints in order they have not applied the previous MK when they are trying to claim the next on
In this situation below you can see that while the MKI Superior Bomber Crew is applied to the unit and the MKII is claimed, the MKIII cannot be claimed until the prior blueprint is equipped to the unit.